Add facet Expressions

[michalhabera]

UFC Expressions are currently cell based, which limits the use of Expressions in codes with boundary data (e.g. a FE function with values depending on facet normal in mixed Poisson, or evaluation of gradients right at the boundary).

[pzuritas]

Hi! Is there any progress on this, or maybe a workaround for setting boundary conditions depending on the facet normal in mixed Poisson?

[jorgensd]

@pzuritas a suggested implementation has been created at: #672