Fix `evalFunctionsSens` for projected area constraint

[A-CGray]

Purpose

The evalFunctionsSens method of the projected area constraint currently fails if it is called when there are no geometric design variables because only the creation of the dAp0/1/2 arrays are skipped, but the rest of the function, that uses those arrays, is not. This is fixed by simply skipping everything if nDV ==0.

Also:

• Altered the projected area constraint to allow for arbitrary projection directions (not just x,y,z)
• Made the sensitivity calculation marginally more efficient
• Split the box projected area test into one test that checks that the projected areas in the x y and z direction match the expected values, and another that tests the derivatives of the projected area in a random direction. The reason is that the sensitivities of the box's projected areas in the x, y and z directions are inaccurate due to the presence of surfaces perfectly orthogonal to the projection direction
• Added some comments in the code explaining this issue with the derivatives
• Switched from using np.random.seed and np.random.rand to np.random.default_rng and rng.random
• Re-trained DVConstraints tests due to new random numbers

Expected time until merged

Type of change

• [x] Bugfix (non-breaking change which fixes an issue)
• [x] 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

Checklist

• [x] 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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.83673% with 4 lines in your changes missing coverage. Please review.

Project coverage is 65.41%. Comparing base (3070c1a) to head (8d199ad). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pygeo/constraints/DVCon.py	73.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #253 +/- ## ========================================== - Coverage 65.42% 65.41% -0.02% ========================================== Files 47 47 Lines 12307 12315 +8 ========================================== + Hits 8052 8056 +4 - Misses 4255 4259 +4 ```

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[marcomangano]

The changes make sense, but I wonder if it is more elegant to have something like:

if nDV == 0:
      pass

[A-CGray]

The changes make sense, but I wonder if it is more elegant to have something like:

if nDV == 0:
      pass

I don't think it matters, this solution would be 1 line longer than what's currently there.

[A-CGray]

@eytanadler so glad we figured this out so now you and @hajdik can get the mphys tests working :)