WIP: Firedrake connection-functional

The goal of this merge request is to supersed this previous firedrakeConnection merge request by providing a better developer interface.

As well as adding some documentation, some tests, and a tentative implementation of exporting a mesh to firedrake, the new structure is as follows:

For firedrake conversion we have functions implementing the following:

• Reference Cell
  • Map between firedrake reference cell and modepy unit coordinates
  • Pull unit nodes from the firedrake reference element (finat element)
  • Generate matrix to flip negative elements
  • Generate matrix to resample a function from firedrake's unit nodes at the unit nodes meshmode is using
• Mesh Generation
  • From a mesh topology produce
    • Nodal adjacency
    • Facial adjacency groups
    • vertex indices
    • Boundary tags
    • boundary tags
  • From a mesh geometry produce
    • vertices/nodes
    • element orientations
    • a mesh element group
    • As well as flip any negatively oriented elements, and handle the consequences with facial adjacency groups etc.

The above functions are then used in import_firedrake_mesh to create a meshmode Mesh and obtain the orientation of each cell in the firedrake mesh. Then, we have only make one new class (for creating a connection from meshmode into firedrake)

class FromFiredrakeConnection:
    """
    .. attribute:: to_discr

        The discretization corresponding to the firedrake function space
    """
    def __init__(self, cl_ctx, fdrake_fspace):
    """make the connection"""

    def from_fspace(self, dim=1):
        """
        Firedrake puts functions mapping into R^m and R^n in different function 
        spaces if m != n, but we want this connection to be able to transport any 
        function on this mesh that has this reference element 
        """

    def from_firedrake(self, fdrake_fntn, out=None):
        """
        Convert a firedrake function and return it, optionally storing in
        array out
        """

    def from_meshmode(self, field, out=None):
        """
        Convert a meshmode field and return it as a firedrake function, optionally
        storing in already instantiated firedrake function out
        """

The class stores the parts of the firedrake function space it needs to implement from_fspace, as well as privately storing the node permutation (fd->meshmode) and the resampling matrix. To implement conversion (fd->mm) one way it just reorders the nodes then resamples them.

Some care is taken to handle "CG" firedrake spaces as well. Conversion fd->mm is easy, conversion mm->fd is disabled by default in this case, but can be forced to work if the user believes they are converting a continuous function. That assumption can also be verified.

TODO: finish exporting discretizations as "DG" firedrake spaces (i.e. the ToFiredrakeConnection class). This seems to be a little easier now that I know most of the big problems. The grubbiest part right now would be boundary tagging. Tests are needed

TODO: more testing is probably good idea on the import side as well, especially related to boundary tagging.