Homogenize Python Bindings for DirichletValues + Support for fixing Subspace Basis

[henrij22]

• [x] homogenize Python Bidnings into fixBoundaryDOFs function
• [x] add properties for size and container to DirichletValues in Python
• [x] add missing interface methods to DirichletValues in Python (fixIthDOF)
• [x] add support for Subspace Basis fixing to DirichletValues into C++
• [x] support subspace basis python bindings with DirichletValues
• [ ] expose interface for inhomogenious DBC to Python (different PR)

This also relates to issue #277

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.63%. Comparing base (1f6d41c) to head (390f2e1).

Additional details and impacted files

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[henrij22]

So this should be more or less ready. I also added the missing bindings for size to the flatassemblers

[henrij22]

@tarun-mitruka This might break some of your existing code, as the function

  void fixIthDof(const MultiIndex i);

is replaced with

  void setSingleDOF(const MultiIndex i, bool flag);

[tarun-mitruka]

@tarun-mitruka This might break some of your existing code, as the function

  void fixIthDof(const MultiIndex i);

is replaced with

  void setSingleDOF(const MultiIndex i, bool flag);

Okay. Thank you for mentioning. You can also add such breaking changes in CHANGELOG.md. I will check out the reasons for this change while doing the review.