gpu - move code to gpu via cupy and numba

[lastephey]

Once we have done the legwork to make gpu_specter gpu-friendly, we can transition the cpu code itself to gpu code. We learned how to do this before and during the March 2020 hackathon so we can apply what we've already learned.

These sections of code include:

• projection_matrix
• hermvander
• legvander
• evalcoeffs
• multispot
• cache_spots
• ex2d_patch

^^ These were all largely cupy or numba-ized during the hackathon. Some cpu parts remain inside cache_spots and evalcoeffs.

Additional functions that should be gpu-ized:

• the if full_output: section of ex2d
• the rest of ex2d