Move nparray conversion to separate function

Move the 2D ndarray to flat C float array conversion used for
ColorMatrix1 conversion to its own function, which will allow it to be
used in other locations.
Support ColorMatrix2, CalibrationIlluminant1, CalibrationIlluminant2

Allow specifying ColorMatrix2, CalibrationIlluminant1, and
CalibrationIlluminant2 DNG tags as arguments.

ColorMatrix2 behaves like ColorMatrix1, except that it will be omitted
if None.  The CalibrationIlluminant fields will only be included if
non-zero.

References #320
All illuminant constants