Add strain contribution to the total energy density

[prkkumar]

We currently ignore the contribution of strain to the total energy density in the TDGL equation. The strain energy can be computed as a function of polarization using equation 4 and 5 in
https://aip.scitation.org/doi/pdf/10.1063/1.1377855

We can add this as follows:

1. Create a new function to compute strain energy with polarization as input.
2. Add the strain contribution to the function that computes TDGL RHS

[jackieyao0114]

The static strain can be solved in a similar way as the electrostatic fields, i.e. solving Poisson equation, except that strain is a 3x3 tensor.
Here is another nice reference: https://www.sciencedirect.com/science/article/abs/pii/S1359645413006617