Set up cofi.utils and create Regularisation utilities

[jwhhh]

Aims to close #54

• BaseRegularisation
• QuadraticReg
• Tests & coverage
• Documentation for classes
• Documentation for methods
• Add findiff into dependency list (docs)
• Review and confirmation on terms within the team

[jwhhh]

Hi @msambridge and @valentineap , could you please help check if the terminologies and documentation make sense?

1. I created a base class BaseRegularisation in case we will have other kinds of regularisation later. Under that, the subclass QuadraticReg is the one we will mostly use (and it's the only subclass now). Is this setup ok / necessary?

2. Please read this documentation page.

   1. Please let me know if terms like "QuadraticReg", "damping", "roughening", "flattening", "smoothing", "byo_matrix" can be made better / more accurate.
   2. Double-checking the formulae presented on this page would be great.
   3. It would also be nice to see if the example usages are intuitively understandable.

3. Please let me know if there's a mistake in implementing the addition of two terms, by looking at the documentation of this function.