More flexible BC specification

[presheaf]

Øyvind proposed to add support for a more mathematical way of specifying BCs. For example, instead of having to look up that bdy0 corresponds to the subdomain x=0 and writing bdy0=g, it would be more convenient (and not much longer) to write u(x=0)=g.

This would give the user a lot of freedom when specifying subdivisions. For example, a 'corner' of the unit square could be defined with u(x<0.5 && y<0.5) . Øyvind suggested using the CompiledSubDomain method to do this, and adding it doesn't seem that hard.