Changing multimaterial json

[shimwell]

Relating to recent issues #135 #134 #133 #129

This PR improves and simplifies the code in a few ways. In general there were some cases where the user could do things in a couple of different ways and it would not be clear which was the final output.

For example:

• setting desity via the density attribtue and density_euqation attribute can lead to conflicting values.
• Making a Material with a name that matches the internal database could lead to unpredicatble results

Removes unused keys in the database for packable and enrichable
Removes density_equation attribute and allows density to accept a string input which is evaluated as an equation
Adds the nmm.Material.from_library() method for clearing making materials from the internal dictionary / database. This can be extented to be enhanced in the future regarding which database to retrieve.
Removes MultiMaterial() class and replaces it with nmm.Material.from_mixutre()

TODO

• The docs will need updating after this goes into the main branch
• new video for the docs
• refactor methods of reading in material files and add tests

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@22be01e). Click here to learn what that means.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##             develop     #136   +/-   ##
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  Coverage           ?   91.44%           
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  Files              ?        3           
  Lines              ?      538           
  Branches           ?        0           
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  Hits               ?      492           
  Misses             ?       46           
  Partials           ?        0           

Impacted Files	Coverage Δ
neutronics_material_maker/utils.py	82.48% <24.00%> (ø)
neutronics_material_maker/material.py	94.31% <84.88%> (ø)
neutronics_material_maker/__init__.py	100.00% <100.00%> (ø)

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