`isotropic_fluorescent_thick_3d` model

[talonchandler]

This PR is an isotropic_fluorescent_thick_3d model that uses torch and our new calculate_transfer_function and apply_inverse_transfer_function structure.

Highlights:

• if you would like to see the fluorescence optical transfer functions and a simple reconstruction, run python examples/models/isotropic_fluorescent_thick_3d.py
• there were a couple natural refactors along the way: a shared padding function that I moved to utils, and a shared forward model between phase_thick_3d and isotropic_fluorescent_thick_3d
• I added NotImplementedErrors to our TV reconstructions, since they haven't been moved to torch yet

@edyoshikun I particularly appreciate your review, since I know you're actively using fluorescence reconstructions.

[talonchandler]

Thanks @edyoshikun and @ziw-liu!