add speckled lasers to LASY

[RTSandberg]

This PR adds speckled lasers to LASY for studying laser-plasma interactions and ICF

[RTSandberg]

Some questions I had:

• The interface is a bit inconsistent between 2-tuples and 2-lists, is there a preference for one v. the other?
• Is there value in using Enums for valid smoothing types?
  I like some a programmatic way of identifying, listing, accessing, and using valid types, but maybe a list of available types is sufficient?
• Is it desirable to just have RPP/CPP, no temporal motion of speckles?
• The equation in readthedocs is too long, is there a way to do multi-line equations?
• The speed of light c is currently hardcoded in at low precision, what is the desired way to include c?
• The testing is not very systematic; are the provided tests sufficient? Is it desirable to be more comprehensive over smoothing types and asymmetric parameter choices in the tests?
• There are certain unused parameters such as cep, z_foc, etc. How do we want to deal with these?

[RemiLehe]

I only had time to go through about half of the PR, but I wanted to already post the comments I had so far.