Add fillet intersection method

[hajdik]

Purpose

Merge the fillet intersection method into the rest of DVGeoMulti. This method allows for individual parameterization of two components (e.g. wing and fuselage) connected by a fillet. The fillet is not parameterized, it follows the other two components using inverse distance warping.

These changes involve some restructuring of existing DVGeoMulti code to take advantage of the shared methods between the original intersection method and this one. This PR will also add a test to the pyGeo tests.

I also made a change to pyGeo's FFD writing that lets you turn off writing the embedded volume so the output file has one less zone and it's easier to view a bunch in tecplot.

I'll add a tutorial to MACH-Aero to document this method once the journal paper is published.

Expected time until merged

No rush

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (non-backwards-compatible fix or feature)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Documentation update
• Maintenance update
• Other (please describe)

Testing

I'm adding my 2D test case as a test for this new code, these new tests should run successfully.

Checklist

• I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation