Show experimental markers from c3d or markers arrays.

[aceglia]

TODO: find a better place to raise error message if number of markers' frame not match number of Q frames

[pariterre]

Hi @aceglia
I made some changes. Now if you want to load the markers data, it should not be passed in the constructor, but using the method load_experimental_data or load_c3d.

I allowed to load a c3d file without loading any model as well, which can be useful to see a C3D (only markers so far)

Thanks for the PR!

[pariterre]

LGTM