flexibilize types for partial_trace

[araujoms]

Currently partial_trace does not accept Hermitian or Symmetric matrices, and always returns a ComplexF64 result, independent of the type of the input. This PR fixes both issues. I'm also adding a convenience method to allow calling it with dim_2_keep being a integer instead of a vector.

I hope you find it useful.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fc2ec75) 80.40% compared to head (ea673d4) 80.36%.

Files	Patch %	Lines
src/math_util.jl	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #109 +/- ## ========================================== - Coverage 80.40% 80.36% -0.04% ========================================== Files 36 36 Lines 2281 2282 +1 ========================================== Hits 1834 1834 - Misses 447 448 +1 ```

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[neversakura]

@araujoms Thanks!