#9 add _do_contour which contains most of contour and contourf
 #21 Nansatmap: proper documentation for Nansatmap.contour(f)
 #21 Nansatmap: simplify pcolormesh
 #21 Nansatmap: remove get_interval and nan2num
 #21 Nansatmap: replace self.d[] with self.
Merge branch 'issue9' into develop
Merge branch 'develop' of https://github.com/nansencenter/nansat into develop