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Changes:
- add `viscid.mfield` that mimics `numpy.mgrid`
- slice-by-value should now be done with imaginary numbers instead of strings encoded like '0.0f'. The string-'f' format will continue to be supported, but imaginary numbers are cleaner for this purpose.
- slice-by-array and slice-by-mask (bool array) are now allowed
- indexing rules got their own page in the docs
- add `viscid.rotation` module for rotation matrix <-> euler angle <-> quaternion conversions
- add `viscid.to_dataframe` and `viscid.from_dataframe` to go between datasets and Pandas DataFrame objects