RFC: Additions to Compressor.jl

[jtravs]

Here are several additions to Compressor.jl

• Max pressure
• Accounting for reduced nonlinearity for single and double gradients (assumes centred, needs docs)
• Max flow rate for gradients (this is the one I find most useful as it puts an upper limit on the core size besides space)
• Circular polarisation (lower nonlinearity but higher intensity handling)

These are very much just suggestions.

This still needs docs and tests (but so does the rest of the code!)