Suggestions for minor improvements to code quality

[MatthijsMars]

Here's a list of minor inconsistencies and improvements that could improve the readability, consistency, interpretability, and re-usability of the code:

In src/dLux/layers/optical_layers.py

• there's FourierBasis, which is a FourierBasisLayer (inconsistency in naming convention of layers)
• The FourierBasis generates the basis automatically whereas the BasisLayer doesn't (inconsistency of behaviour) I can see why but it is a bit confusing. Also if there's a FourierBasis class that generates them automatically you should maybe also have a Zernike that generates it.
• In the FourierBasis it is not easy to scale the basis (should have a scale parameter that is passed to dlu.fourier_kernels( )
• The FourierBasis doesn't have the as_phase option which all the other layers do.
• There should be amplitude bases as well, could just be by having the option of as_amplitude in addition to as_phase. (or have an as={'opd', 'phase', 'amplitude'})

Happy to implement these myself, just let me know if there's any points you don't agree with and don't want to change.

If I find more things I'll add to this