Fenics-ufl

- Mixed dimensional domain support

- Remove scripts
- Remove LaTeX support (not functional)
- Add support for complex valued elements; complex mode
is chosen by `compute_form_data(form, complex_mode=True)` typically
by a form compiler; otherwise UFL language is agnostic to the choice
of real/complex domain

- Remove python2 support

- Add geometric quantity `CellDiameter` defined as a set diameter
of the cell, i.e., maximal distance between any two points of the
cell; implemented on simplices and quads/hexes
- Rename internally used reference quantities
`(Cell|Facet)EdgeVectors` to `Reference(Cell|Facet)EdgeVectors`
- Add internally used quantites `CellVertices`,
`(Cell|Facet)EdgeVectors` which are physical-coordinates-valued;
will be useful for further geometry lowering implementations
for quads/hexes
- Implement geometry lowering of `(Min|Max)(Cell|Facet)EdgeLength`
for quads and hexes

- Change PyPI package name to fenics-ufl.

- Add the `DirectionalSobolevSpace` subclass of `SobolevSpace`. This
allows one to use spaces where elements have varying continuity in
different spatial directions.
- Add `sobolev_space` methods for `HDiv` and `HCurl` finite
elements.
- Add `sobolev_space` methods for `TensorProductElement` and
`EnrichedElement`. The smallest shared Sobolev space will be
returned for enriched elements. For the tensor product elements, a
`DirectionalSobolevSpace` is returned depending on the order of the
spaces associated with the component elements.