Add mesh operators

[mwarusz]

This PR adds a couple mesh operators: gradient, divergence, curl, and tangential velocity reconstruction. A test comparing the result of applying the numerical operators to an analytical solution on a planar hexagonal mesh is added.

Additionally, this PR uses CMake to automatically download the meshes needed for running the tests. It also fixes a couple of bugs in the horizontal mesh class and tests, that showed up only when running on a gpu.

This is a work in progress, but early feedback is welcome.

Checklist

• Documentation:
  • Design document has been generated and added to the docs
  • User's Guide has been updated
  • Developer's Guide has been updated
  • Documentation has been built locally and changes look as expected
• Testing
  • CTest unit tests for new features have been added per the approved design.
  • Unit tests have passed. Please provide a relevant CDash build entry for verification.

[mwarusz]

Closing to change the branch name to conform to E3SM conventions. Superseded by #62.

[xylar]

@mwarusz and @philipwjones, since develop is considered to be a long-lived feature branch, do we need to conform to the E3SM branch naming conventions? Not that I mind -- I think they're useful -- but maybe it's overkill to enforce them.

[xylar]

Ah, just saw #60 (review). So I guess we want to follow the rules.

[philipwjones]

@xylar Yeah, if nothing else, keeping it here so that when this does become part of E3SM we haven't lapsed into bad habits. Note that since we're typically using our own forks, the username is not required though a more clear name sometimes helps if the github username isn't obvious.