Add equality testing

[jmeyers314]

Can we add __eq__ methods to Piff objects? I wanted to write a test over in Rubin-land that roundtrip serialization to FITS (using the Rubin framework) works, but was stymied by the inability to check if the original and retrieved SimplePSF objects were actually the same. For now I can just check if some sample PSF images come out the same, but it'd be nice to just assert original_psf == roundtrip_psf.

[rmjarvis]

Sure. Fine by me.