v1.2.0: Better unstructured grid support and ready for python 3.7

The 1.2.0 release contains better functionalities for handling unstructured data and it contains some minor bug fix to make it usable for python 3.7.
The 1.2.0 release is the last one for python 2.7. The next releases will only be available for python 3.5+.

Added

• The psyplot.plotter.Plotter.initialize_plot method now takes a priority keyword to only initialize only formatoptions of a certain priority

Removed

• The installers from the psyplot-conda repositories have been depreceated. Instead, now download the latest miniconda and install psyplot and the plugins via conda install -c conda-forge psy-maps psyplot-gui psy-reg

Changed

• We generalized the handling of unstructured data as lined out in issue#6. The new method psyplot.data.CFDecoder.get_cell_node_coord returns the coordinates of the nodes for a given grid cell. These informations are used by the psy-simple and psy-maps plugins for displaying any unstructured data. See also the example on the visualization of unstructured
  grids
• We removed the inplace parameter for the CFDecoder methods since it is deprecated with xarray 0.12 (see issue #8). The CFDecoder.decode_ds method now always decodes inplace