Fix bug in signal-array (max number of signals was 20)

Fix bug in signal-array that was limiting the number of signals in a signal-array to 20. Moreover I replaced the C array with an std::vector.

Fix visibility issues

- No need to export template functions
- Expose some template instantiations

- Replace DYNAMIC_GRAPH_DLLEXPORT by DYNAMIC_GRAPH_DLLAPI
- Remove an unnecessary attribute

Add a method to check if a signal with the given name exists.

Correct the name of the method isPlugged.

The old one is kept, but with a warning message
(for now).

[skip ci]