Thermo-electro-elastic pump

[jppelteret]

Implementation of a problem with thermo-electro-mechanical coupling

The electro-mechanical and thermal problems are solved in a staggered manner.
Two implementations of the electro-mechanical problem are implemented:

• the standard two-field formulation using the electric scalar potential and a
  displacement field (valid for compressible materials), and
• a four-field formulation, extending the above with an additional dilatation
  and pressure field (valid for both compressible and near-incompressible
  materials).

[jppelteret]

This is still a WIP, so please do not review or merge this yet.