[ipopt-iface]: several small changes:

- variable renaming
- use of `assert_pretty`
- add `namespace crocoddyl`
- use of `model&` and `data&` to access problem model and data
- remove ipopt legacy comments
6 jobs for pr/1020 in 22 seconds (queued for 1 second)
Name Stage Failure
failed
doc-coverage Test
src/core/solvers/box-qp.cpp                  142       0     0%   15-16,31,33-34,36-37,39-40,44-51,54-60,64,66,68-69,72-73,76-77,80-81,84-85,90-91,95-97,99-107,109,114-127,129-132,135-136,138-141,143-147,149-154,158-162,164-167,172-174,177-178,181,183,185,187,189,191,193,195-204,207,209-211,214,217-219,222,225-227,230,233-236,238-241,244-245,248,250
src/core/solvers/ddp.cpp 363 0 0% 16,25-26,28-31,33-34,36,40,42,44-46,48-49,51-53,55-56,58,60-61,64-69,71,74,76,79-81,84-86,88,90-96,101-102,104-108,111,113-116,119-121,124-125,128-131,133-134,137-141,144-147,149-152,155,158-166,169,172-174,176-189,191,194-197,199,201-203,206-210,212-213,216-217,219-226,228-252,256,258-265,267-268,270-271,275-276,279-280,282-283,286,289-291,294-301,303-308,310,312-313,315-317,319-321,325-328,330-332,334,337-347,349,351-355,357,360-363,365,368-371,373,376-386,388-390,392-394,396-409,411-412,414,416-417,419-421,423-426,428-429,432,434,436,438,440,442,444,446,448,450,452,454,456,458,460,462,464,466,468,470,472-474,477,480-482,485,488-490,493-494,497-499,502,505-507,510,513-515,518,521-523,526,529-532,534-537,540-541,544,546,549-551,554,557-559,562,565-567,570
src/core/solvers/fddp.cpp 165 0 0% 18-19,21,23,25-27,29-30,32-34,36-37,39,41-42,45-50,52,55,57,60-62,65-67,69-70,72-78,81-86,91-92,94-98,101,103-106,109-111,114-115,118-121,126-129,131-134,137-139,142-149,151-156,158-161,166-168,171-181,183-187,189,191-192,194-196,198-200,204-208,210-212,215-223,225,227-228,230-232,234-236,240-244,246-248,251,254,256-258,261
------------------------------------------------------------------------------
TOTAL 1106 0 0%
------------------------------------------------------------------------------
$ gcovr -e CMakeFiles -e _deps -r . --html --html-details -o ${CI_PROJECT_DIR}/coverage/index.html
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
robotpkg-py-crocoddyl-buster Test
| make[1]: *** [CMakeFiles/Makefile2:76: CMakeFiles/crocoddyl.dir/all] Error 2
| make: *** [Makefile:144: all] Error 2

For details or bug reports, check the complete log file in:
/root/robotpkg/optimization/py-crocoddyl/work/build.log
make: *** [/root/robotpkg/mk/build/build.mk:199: do-build] Error 2
make: *** [/root/robotpkg/mk/internal/trusted.mk:36: install] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
format Test
+  std::vector<Eigen::VectorXd> us_guess_;

std::size_t nx_;
std::size_t ndx_;
++ git diff --ignore-submodules
++ wc -l
+ exit 15
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
robotpkg-py-crocoddyl-ferrum Test
For details or bug reports, check the complete log file in:
/root/robotpkg/optimization/py-crocoddyl/work/build.log

/root/robotpkg/mk/build/build.mk:198: recipe for target 'do-build' failed
make: *** [do-build] Error 2
/root/robotpkg/mk/internal/trusted.mk:36: recipe for target 'install' failed
make: *** [install] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
robotpkg-py-crocoddyl-20.04 Test
| make[1]: *** [CMakeFiles/Makefile2:1518: CMakeFiles/crocoddyl.dir/all] Error 2
| make: *** [Makefile:144: all] Error 2

For details or bug reports, check the complete log file in:
/root/robotpkg/optimization/py-crocoddyl/work/build.log
make: *** [/root/robotpkg/mk/build/build.mk:199: do-build] Error 2
make: *** [/root/robotpkg/mk/internal/trusted.mk:36: install] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
robotpkg-py-crocoddyl-18.04 Test

For details or bug reports, check the complete log file in:
/root/robotpkg/optimization/py-crocoddyl/work/build.log
/root/robotpkg/mk/build/build.mk:198: recipe for target 'do-build' failed
make: *** [do-build] Error 2
/root/robotpkg/mk/internal/trusted.mk:36: recipe for target 'install' failed
make: *** [install] Error 2
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1