Inherit default grid style from matplotlib.rcParams

Based on `matplotlib.axis.YTick._get_gridline`. Note that Matplotlib
implements gridlines with `Line2D`, whereas wcsaxes implements gridlines
with `PathPatch`. The two have slightly different option styles
regarding dashes, so we have to map between them.
Inherit default tick style from `matplotlib.rcParams`

Note that Matplotlib's defaults dictionary contains default settings
only for `xtick.*` and `ytick.*`. Because there is not a fixed mapping
between WCS axes and Matplotlib `x` and `y` axes, the best we can do is
arbitrarily pick one axis from which to inherit defaults.

This is explained in the docstring.
Add unit test for default grid and tick style
Add argument to set tick width

And update original unit tests to explicitly set tick width and size,
overriding default values, so that they continue to pass.
made a new datasets module
functions return specific images now
cleanup
renamed cube function
datasets module now returns hdu data for the images
removed code in docs that used msx.fits and deleted the fits file
Latest updates from Astropy affiliated package template
changed import words, edited documentation accordingly
Merge pull request #51 from astrofrog/update-from-template

Latest updates from Astropy affiliated package template
refactored l1448_co_hdu
added option for setting cache in datasets module
Merge pull request #50 from anizami/datasets-modu

For issue #48
Merge pull request #49 from lpsinger/rcparams

Inherit default grid and tick style from `matplotlib.rcParams`
spacing has to be quantity for ScalarFormatterLocator
added format_unit to change units
oops forgot to remove a * self._format_unit