mark user-facing methods as const where possible

[joyal-mathew]

User-facing methods should be marked as const where possible to allow more flexibility in user code.

For example, MeshField::OmegahMeshField::triangleLocalPointEval. In my use case, I need to be able to call this function from a const context.

[cwsmith]

IIUC, you need the function to be const as opposed to the return type?

[joyal-mathew]

Yes, since I'm calling it from another const method.