Esmf conserve

This is just a first attempt.
It adds an experimental routine to perform conservative regridding (i.e. preserving area sums).
It uses the ESMPy interface to the ESMF codebase ( See notes at www.earthsystemmodeling.org/ ).
It *should* work once you have installed ESMPy.  I have a site-specific script for that, if you need it.
 -- or see how it is done in '.travis.yml'

More importantly, what it does not yet do ..
  * no support for curvilinear (2d) coordinates
  * some known problems with odd testcases, which for now have known-failures coded into the tests (cf. test_global_collapse and test_single_cell)
  * some other odd testing cases seemed to cause low-level ESMF errors.  May need work to identify certain specific problem cases + raise more intelligible warnings

More testcases maybe needed :
  * irregular grids