Replace eval with asteval (#97)

[DanShort12]

• Introduce asteval as a dependency

• Use an asteval interpreter

Make sure the temperature and pressure can be set in the symbol table. Raise error if asteval detects invalid input. Rework checks for OpenMC being available, allowing exceptions to be raised if OpenMC materials aren't created successfully when OpenMC is installed. Implement a test to check that arbitrary code in the density_equation results in an error.

Co-authored-by: Jonathan Shimwell jonathan.shimwell@ukaea.uk Co-authored-by: autopep8 autopep8@users.noreply.github.com

[codecov[bot]]

Codecov Report

Merging #98 into main will increase coverage by 0.28%. The diff coverage is 82.35%.

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   90.87%   91.15%   +0.28%     
==========================================
  Files           3        3              
  Lines         515      520       +5     
==========================================
+ Hits          468      474       +6     
+ Misses         47       46       -1     

Impacted Files	Coverage Δ
neutronics_material_maker/material.py	89.91% <82.35%> (+0.42%)	:arrow_up:

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[shimwell]

Thanks Dan