add is_hermitian to Matrix and SparseMatrix

There are several `is_*` methods for matrices:

```
is_anti_symmetric
is_diagonal
is_diagonalizable
is_hermitian  <---- this is being added in this PR
is_lower
is_lower_hessenberg
is_nilpotent
is_square
is_symbolic
is_symmetric
is_upper
is_upper_hessenberg
is_zero
```

The `is_hermitian` defined here is not part of the assumptions system since Matrix doesn't subclass from Expr but it gives the same sort of answer: True, False or None.

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