Add optional entity argument to the tabulate method

This pull request adds an optional entity argument to the FiniteElement.tabulate method. This new argument specifies the sub-entity of the reference cell to tabulate on, for example the cell itself (default) or a particular facet. In general, we expect the coordinates to be in entity-coordinates; i.e. facet coordinates for facet tabulation or cell coordinates for cell tabulation.

The entity argument is represented as a pair of the form (d, i), where d is the dimension of the sub-entity, and i its corresponding index. For example, the cell has an entity pair (dim, 0). For a reference triangle, there are three facets with entity pairs: (1, 0), (1, 1), and (1, 2).

This PR is a first step towards a reliable implementation of the trace element, where facet ids are known and provided by the form compiler. See a draft implementation here. FInAT also relies on this new API.