Interpolation in pde

[mkstoyanov]

We can now use interpolation to handle some non-separable cases:

• non-separable initial conditions
• non-separable exact solution (for verification purposes)
• non-separable forcing terms
• non-separable mass terms

Still missing (will come soon):

• second and higher order basis, works only with linear basis
• cannot take derivatives of the operators, works only with the mass term
• cannot do Hermite interpolation
• support for local kronmult and MPI