Flexibility for parametric dimension of loads (Closes Issue #20)

[hverhelst]

(See Issue #20)

[hverhelst]

A nicer fix would be when we have a gsCompositeFunction that takes a gsFunction and performs the composite map via the geometry map automatically.

In that case, we can make gsFunctionSet<T> * m_surfForce; a class member, and assign it in the constructor of the gsThinShellAssembler like (DRAFT):

if (par) // function is defined in parametric domain
    m_surfForce = new gsFunctionSet<T>(surface_force);
else
    m_surfForce = new gsCompositeFunction<T>(surface_force,m_patches);