Add Unstructured Grid class

[fluidnumerics-joe]

Properties and procedures that are common to unstructured grids and structured grids are defined in the BaseGrid class. This includes lon,lat, time, time_origin, and mesh properties.

The Grid class is now defined as a type extension of BaseGrid. This configuration is compatible with the previous commit of Parcels (all existing tests pass). The _zonal_periodic, _zonal_halo, _meridional_halo, and _lat_flipped properties are seen as attributes specific to the structured grid Grid class.

The UGrid class is defined as a type extension of the BaseGrid. This child class adds the face_node_connectivity property that relates vertices to the 2-D lateral grid faces. The lon and lat attributes refer to the node_lon and node_lat that are defined in the uxarray.Grid structure; these are the corner node vertices. Note that there are other possible nodes, including the edge vertices (centers of the edges), and face_vertices that define the element centroids; these are currently not defined.

[fluidnumerics-joe]

@erikvansebille @VeckoTheGecko I'm using this PR to provide visibility to some of the proposed changes to Parcels to support unstructured grids. So far, I've only inserted a BaseGrid class that the Grid and UGrid are child classes of. All of the tests under tests/ pass, which is good.

From the state it's in, I'll need to work on a creategrid method for the UGrid class and I suppose it'd be ideal to sort out how we'll bring in UXArray support for loading the grid.

This PR is meant to go to the ELPHE-WW Fork's main branch so that we have a safe "playground" to explore.

Your comments and feedback are most welcome before further changes are made.

[erikvansebille]

Thanks @fluidnumerics-joe, for starting this work. Your plan sounds great to me. I quickly went through the code, and that also looks fine. Some issues to fix with the CI obviously, but I don't think these are huge problems

[VeckoTheGecko]

Looks good!