Pygeogrids

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- fix bug in lookuptable generation when gpis have custom ordering
- add functions for getting subgrids from cells and gpis

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- change meaning and rename grid dimensions to lon2d, lat2d. They do
now represent 2d arrays of latitudes and longitudes which means that
they no longer have to be regular in order to be able to have a
shape. This is useful for e.g. orbit data

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- fix issue 19 by refactoring the iterable checking into own function
- made pykdtree an optional requirement see issue 18

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- added support for saving more subsets and loading a certain one
in/from a netcdf grid file
- fix 15 by setting correct shape for derived cell grids
- fix issue 14 of gpi2rowcol input types

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- Initial version pulled out of pytesmo
- added support for iterables like lists and numpy arrays to functions
like find\_nearest\_gpi. numpy arrays should work everywhere if you
want to get information from a grid. see issue 3 and 4
- fixed bugs occuring during storage as netCDF file see issue 8
- comparison of grids is no longer using exact float comparison, see
issue 9
- added documentation and examples for working with the grid objects,
see issue 1