Add base_ring_type for MatrixElem; fix it for MatSpace

[fingolfin]

This is a useful "inverse" for dense_matrix_type.

[codecov[bot]]

Codecov Report

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

Project coverage is 87.28%. Comparing base (964e8c9) to head (5fa8233).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1705 +/- ## ======================================= Coverage 87.27% 87.28% ======================================= Files 116 116 Lines 29721 29722 +1 ======================================= + Hits 25940 25942 +2 + Misses 3781 3780 -1 ```

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