ocean_model_grid_generator submodule to NCTools

[ngs333]

This pull request adds the ocean_model_grid_generator project ( https://github.com/NOAA-GFDL/ocean_model_grid_generator ) as a submodule to NCTools. One key goal is to have the
grid generator appear in as much as possible as just another tool of NCTools. To that end : the
grid generator has its own subdirectory under FRE-NCtools/tools; the grid generator (although it is a python script)
is installed along with the other NCTools scripts; and unit tests were created within NCTools that are in the same
directory as all the other NCTools unit tests, and are run by the CI.

There are two issues remaining:

1. One of the four unit tests is commented out as the expected output is not produced. This is currently under investigation.
2. The coordination between the two repositories is still TBD. With this proposed merge it is partly manual in the sense
   that the NCTools subproject managers would only know of a problematic change in the submodule source repo if either a unit test fails, or if a submodule source repo developer informs us of a change. Of course, the submodule source repo is recursively cloned anew and tested every time any CI run is made, but is this enough?