New methods for Input class

[AlbertoBittes]

Description

Two new methods (with tests):

• add material (and density) to void cells.
• add fill=/u= parameter keyword to cells that don't have it.

Additional dependencies introduced: None

Type of change

Please select what type of change this is.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature
  • [x] Non-breaking change which adds functionality
  • [ ] Breaking change fix or feature that would cause existing functionality to not work as expected

Other changes

• [ ] This change requires a documentation update

Testing

• test_add_material
• test_add_cell_fill_u

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] General testing
  • [ ] New and existing unit tests pass locally with my changes
  • [ ] Coverage is >80%

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.66667% with 6 lines in your changes missing coverage. Please review.

Files	Patch %	Lines
src/f4enix/input/MCNPinput.py	86.66%	6 Missing :warning:

Files	Coverage Δ
src/f4enix/input/MCNPinput.py	94.93% <86.66%> (-0.49%)	:arrow_down: