v1.2.0: Improved visualization of unstructured grids

This new release, inline with the psyplot-1.2.0 release adds better support for unstructured grids.

Added

• The mapplot plotmethod now also supports unstructured data of any shape (see issue#6)

Changed

• The lonlatbox formatoption now selects a subset of the unstructured data to lower the size of the data array. Previously, data points outside the specified lonlatbox where simply set to NaN