I added scaf_system(), which performs the same setup as scaf_solve(), but instead of performing the actual solve and line search, it returns the linear system. Also exposed scaf::compute_energy() in the public interface.

This is useful when the user code optimizes a sum of the SCAF energy and some other energy.

Added a test (scaf.cpp) that calls the new function, performs the solve and line search manually and then compares the result to the original scaf_solve().

Check all that apply (change to [x])

• All changes meet libigl style-guidelines.
• Adds new .cpp file.
• Adds corresponding unit test.
• This is a minor change.