Started grid class that works with cartopy
Added interpolation and grid generation
Better plots and check array sizes

Tried meshgrid non-copying but a copy is made anyway when .ravel is
called.
Added pcolor and contour

Moved things to separate functions. Need masked arrays for pcolormesh.
Pass projection in regular and scatter

Tell the colorbar which axes to plot to. Add examples of multiple
projections. Examples of scatter plotting with projections. Example of
pcolor and contour plotting with projections.
Load data from .gdf files

Added two sample data files from
http://icgem.gfz-potsdam.de/ICGEM/potato/Service.html

1. go_cons_...: GOCE based gravity model calculated at const. height
2. eigen_...: Mixed-data gravity model calculated on the topography.
Add link to whole dir  in nbviewer