Export datasets for simulation with PDEnsorflow

[p-j-smith]

We should support exporting openep datasets in a format that can be used for running monodomain simulations with PDEnsorflow

To do this we need to:

• Convert a surface mesh into voxel representation (either using thickness data from e.g. Kodex or assuming homogenous thickness)
• Add fibre orientations
• Define cell models
• Export data in nifti format (or other suitable format)