adds default stp filenames based on name

[shimwell]

Proposed changes

Adds default stp filenames for reactor.export_stp(). If filename is left as None then the shape.name will be used as the stem of the stp file. This makes it easier to export the stp file for a reactor when users don't nessecarily know the number or order of components in the reactor

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Pep8 applied
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov]

Codecov Report

Attention: Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.

Project coverage is 96.78%. Comparing base (02c89e7) to head (ce8e3a3).
Report is 813 commits behind head on develop.

Files	Patch %	Lines
paramak/reactor.py	56.52%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #107      +/-   ##
===========================================
- Coverage    96.84%   96.78%   -0.07%     
===========================================
  Files           76       76              
  Lines         4724     4728       +4     
===========================================
+ Hits          4575     4576       +1     
- Misses         149      152       +3     

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