feat: inner products for mixed method

[chauj96]

This PR adds the implementation of computeM, which computes the inner product matrix $M$ used in the MFD mixed method formulation.

1. The function has been implemented for the following inner product types:

• TPFAInnerProduct
• BdVLMInnerProduct
• QuasiTPFAInnerProduct
• SimpleInnerProduct
  Each class now includes a computeM routine that construct the inner product matrix $M$ required for the mixed formulation.

2. Testing
   Unit tests were added in testMimeticInnerProducts to verify the correctness of the newly implemented computeM functions. All tests currently pass.

+) Minor variable naming updates were applied in SimpleInnerProduct to keep the geometry nomenclature consistent with the other inner product implemenations.