Create class to represent MHD normalizations

[namurphy]

This PR will add a class that represents a particular system of normalizations that is commonly used for the equations of MHD, in particular for simulations and theory where we can't be bothered to deal with units. I'm just getting started on this, but will probably use some commits that I had temporarily had in #859, though I ultimately decided to reduce the scope of that PR to an abstract base class. This partially addresses #808, but there are other common normalizations that will need to be added (hence the abstract base class).

At the moment I'm thinking that it would be good to allow the inputs to be Quantity instances with the value of the normalization coefficient, Unit instances (for example if we want to normalize length to 1 * u.mm, or a ParticleLike object (since the choice of ion matters when going between mass density and number density). It'd also be good to be flexible in what inputs the user may provide. I usually set L, n, and B, but others might normalize to different things. I'm also wondering if there would be a good way to allow users to specify that the function to normalize length to would be, e.g., the ion inertial length or gyroradius.

[codecov]

Codecov Report

Attention: Patch coverage is 75.89286% with 27 lines in your changes are missing coverage. Please review.

Project coverage is 94.96%. Comparing base (2a6cf7f) to head (85b1a92).

Files	Patch %	Lines
src/plasmapy/simulation/normalizations.py	74.52%	24 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
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- Coverage   95.18%   94.96%   -0.23%     
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  Files         103      104       +1     
  Lines        9412     9524     +112     
  Branches     2153     2182      +29     
==========================================
+ Hits         8959     9044      +85     
- Misses        276      300      +24     
- Partials      177      180       +3     

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