Full affine atom implementation to support CVXPY

[Transurgeon]

We are missing a few affine atoms that are supported in CVXPY.
This issue will serve as a tracker to eventually add them in.

• left kron
• right kron
• conv (convolution)
• diag_mat
• upper_tri (for PSD constraints)
• vstack

Eventually it would be nice to support everything, such as ND expressions and general broadcasting
but that's probably a lot of work and won't be necessary for now.

[Transurgeon]

diag_mat and upper_tri can be implemented by getting the indices and calling make_index. This might be slightly less performant, but it keeps things cleaner for now.

[dance858]

Nice, thank you for making this list.

[Transurgeon]

made a PR for vstack in #52

[Transurgeon]

implemented diag_mat and upper_tri through index in #54

[Transurgeon]

I'll close this since we recently added Kron and have full support for cvxpy atoms now!