Make pyvisgen dependency optional

[tgross03]

• Made pyvisgen dependency optional and moved import to if TYPE_CHECKING clause to only import it for type hinting

Fixes #5

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 45.40%. Comparing base (9c90c2f) to head (c5ef9c4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pyvisgrid/core/gridder.py	50.00%	1 Missing and 1 partial ⚠️
pyvisgrid/core/stokes.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
- Coverage   45.61%   45.40%   -0.22%     
==========================================
  Files           7        7              
  Lines         331      337       +6     
  Branches       53       55       +2     
==========================================
+ Hits          151      153       +2     
- Misses        174      176       +2     
- Partials        6        8       +2     

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