Added convert_units(), plus mod to allow arithmetic operations on compatible units and conversion of arrays of integers.
Review actions.
Abbreviated numpy namespace

All modules now use:
    import numpy as np
Abbreviated masked array namespace

All modules now use:
    import numpy.ma as ma
Numpy namespace abbreviations in documentation
Removed arithmetic enhancements to avoid issues with temperature anomolies. Docstring improvements.
Merge pull request #334 from esc24/convert_units

Added convert_units() to cubes and coords.
Numpy namespace abbreviations in test code
human readable datetimes and cube unit printout
Merge pull request #338 from ajdawson/numpy-namespace

Standardising numpy namespaces
Merge pull request #323 from cpelley/print_units

Improved str() support: Cube units, and human-readable dates.
Changes to cartography.py, copied from PR.212
Small style + readability fixes.