Force dimensions

[hverhelst]

It would be nice to have more flexibility with the dimensions of the forces defined.
The assembler assembles the surface force always like this

auto m_force = m_assembler.getCoeff(*m_forceFun, m_ori);

This means that m_forceFun should have the same domain dimension as the geometry map m_ori.

It would be nice to make this more flexible. For example, when we have a gsFunctionSet-derived class that computes the composition, we can register m_forceFun using this composition, and keep the rest of the code the same.