400 backward euler solver option

[K20shores]

Closes #400

• adds begin, end iterators to the matrix class so that we can do math on any type of matrix, needs to be added to vectorized matrix
• creates a backward euler solver, but only works for our regular matrix type and isn't entirely tested

more tasks to finish this completely are listed in #488

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 77.38095% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 92.90%. Comparing base (9c86cc8) to head (b65268e).

Files	Patch %	Lines
include/micm/solver/backward_euler.inl	75.00%	19 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #485 +/- ## ========================================== - Coverage 93.32% 92.90% -0.42% ========================================== Files 40 42 +2 Lines 3177 3258 +81 ========================================== + Hits 2965 3027 +62 - Misses 212 231 +19 ```

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