…ytroll#1486 PR

This is done because the reader name in polar2grid is mirs, and that
has been used for a while, so it should not change for users

Remove variables that are never used
There seems to be no reason to handle coords again in __getitem__,
let xarray handle these.

1.)  Do not need the file registry before running pooch retrieve command
2.)  Simplify/make easier to read code that does the pooch retrieve
     and call when needed, not at the beginning of the code. The filenames
     do not need to be in the metadata for the scene.
3.)  Use dask.where rather than roundabout numpy logic.  This eliminates
     all squeeze calls.
4.)  removes bt_data array slice where it is not needed but does use the
     bt_data attributes for the xarray of the corrected brightness temperatures

limb_correct_bt does not need to be in the class.  Move
code out of class methods and restructure the call to the
limb_correction to facilitate this move.

Assigning a map_blocks call to a numpy array 96 times
causes dask to compute before it is necessary.  Also, I could be wrong,
by it does not seem necessary to loop over map_blocks.  It makes more
sense to loop over the fov in the function that map_blocks calls.

Merge branch 'master' of https://github.com/pytroll/satpy into mirs

…py arrays

also, don't hard code the block size for the arrays, use the block size of the input daskArray.

…ays.

1.)  Remove extra check for self.nc.coords and just check within new_coords
     method
2.)  Change all dask calls in apply_limb_correction method to
     np calls.

This feature of xarray is not working well with dask
pydata/xarray#3068

1.)  Removes the new_coords method which added lat/lon coords
     through xarray.Dataset.assign_coords method.
2.)  Removes coordinate assignment when bt_corrected xarray.DataArrays
     are constructed.
3.)  Restructures available_datasets:  Hope, it is easier to understand
     and ensures that lat/lon coordinates have standard_names even
     if file metdata without it takes precedence after the yaml.

   otherwise they are dim_0 and dim_1 and they are not recognized by
   the reader.  Scanline/FOV are converted to y,x in reader

The necessary metadata to get metop files read into satpy
is getting added in the reader.  The information provided to
the yaml would be nice to supplement information in the file, but
I can't think of a reason why it should override information, except
for descriptions, which can be handled elsewhere.  Since trying to
juggle yaml/file metadata would be a little more involved in the code,
I feel it is best to keep the model of the gaasp reader which assumes
the necessary metadata is in the file and add missing metadata within the
reader.

- add the mocks
- create fake coefficient data in test
- updates how the coefficients are read in mirs by splitting our the
  reading of the file and the actual parsing.
- Checks the end of coefficient file for n_chn and n_fov thus removing
  the need for globals
- Remove the global N_FOV in the loop which calculates the coefficients
  on the data.  Use dataset size for loop endpoint, if the dataset
  for some reason does not have 96 fov, this loop would fail with a
  fixed endpoint.