fix bug + regression test
basic implementation of 1D *spectral* slices (WCS still has trouble with
non-spectral 1D slices)
fix a call to _new_cube_with and add a .value property to return a
unitless copy of self
implement @ChrisBeaumont's comments
Merge pull request #156 from keflavich/preserve_float_bugfix

Minor bugfix regarding nansum
Merge pull request #155 from keflavich/preserve_unit_2

Preserve the spectral units when creating an output header: rd. 2 - refactor!
REMOVE np.asarray: it does bad things to Quantities!
whitespace
asarray for value.ndim!  THAT's how you do it... riiight.
Merge pull request #157 from keflavich/slice_1d

Allow 1D Spectral Slices
first attempt at a to_glue machine.  Required fixing some bugs in
__getitem__ but that's no problem
the glue_app becomes part of the cube now so that subsequent calls add
the dataset to an existing session
allow a glue application to be specified.  add docstrings
attempt to accept updates of data collections
have to add cubes to the dataset object, not the datacollection
need to check component labels, not 'components'
autoload glue viewer
add start_gui keyword