[CMake] put python header in include

Otherwise, this header is installed if BUILD_PYTHON_INTERFACE, and not
if INSTALL_PYTHON_INTERFACE_ONLY.

But if we want to be able to have binary packages for:
- the main curve package
- its bindings for python 2
- its bindings for python 3

this header will be installed in the 2 last cases, which will conflict.

In short: if we set INSTALL_PYTHON_INTERFACE_ONLY, there should be only
stuff installed in PYTHON_SITELIB.
20 jobs for v0.3.2 in 19 minutes and 35 seconds (queued for 5 minutes and 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #63711
doc-coverage

00:04:06

failed #63712
allowed to fail
format

00:00:14

passed #63694
robotpkg-curves-16.04-debug

00:01:38

passed #63693
robotpkg-curves-16.04-release

00:01:21

passed #63696
robotpkg-curves-18.04-debug

00:01:13

passed #63695
robotpkg-curves-18.04-release

00:01:25

passed #63698
robotpkg-curves-stretch-debug

00:01:51

passed #63697
robotpkg-curves-stretch-release

00:01:17

passed #63701
robotpkg-py-curves-16.04-debug

00:02:55

passed #63702
robotpkg-py-curves-16.04-release

00:02:39

passed #63705
robotpkg-py-curves-18.04-debug

00:03:25

passed #63704
robotpkg-py-curves-18.04-release

00:03:06

passed #63699
robotpkg-py-curves-py3-16.04-debug

00:04:15

passed #63700
robotpkg-py-curves-py3-16.04-release

00:03:34

passed #63703
robotpkg-py-curves-py3-18.04-debug

00:05:10

passed #63706
robotpkg-py-curves-py3-18.04-release

00:04:27

passed #63710
robotpkg-py-curves-py3-stretch-debug

00:03:35

passed #63709
robotpkg-py-curves-py3-stretch-release

00:15:26

passed #63708
robotpkg-py-curves-stretch-debug

00:15:48

passed #63707
robotpkg-py-curves-stretch-release

00:15:41

 
Name Stage Failure
failed
format Test
     error = true;
std::cout << "SE3 curve : rotation curve not equal to se3.rotation" << std::endl;
}

-
// check if errors are correctly raised
try {
cBezier(0.1);
ERROR: Job failed: exit code 1