Pyminc

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* don't install `test/` dir into site-packages directory

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* drop support for Python 2 and Python3 < 3.5
* don't create a 'test' package in site-packages/

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* added functionality for accessing linear transform component of a general transform,
and associated small bugfixes (added by John Sled)
* fix some failing/broken tests
* one instance of indexing numpy array with a list was changed to use a tuple due to numpy API
deprecation (see e.g. MICe-lab/18)

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* turns out that for memory/time efficiency, you really want to load a transformation grid into memory
only once and apply it to as many coordinates as you can. So added the ability to do just that:
transform_multiple_xyz_coordinates_using_xfm() accepts arrays of x,y,z coordinates

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* added support for transforming x,y,z coordinates using an xfm file: transform_xyz_coordinates_using_xfm()

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* bug fix related to dtype="float". The data of the pyminc object was cast to be "float64" which is
double.