Presentation fin laura

[lasofivec]

Following changes:

• update on the cython speedup notebook
• added small notebook on the computation of the solid angle subtend by a triangle
• added tex files on the computations of said solid angle (my version and Didier's, different notations)
• small changes to better test if a string starts or end with a sub string
• added reference for the computation for the determination if a 3D angle is reflex or not
• better print for errors on _vignetting_tools.pyx

[lasofivec]

Just waiting for the tests to pass before updating the PR text (sorry for the messiness)

[lasofivec]

Update done, ready when you are @Didou09