Replace Integral with PhysIntegral in MultiRegions
We currently have two separate integral methods which essentially carry out the same operation: Integral and PhysIntegral. In essence;
- Integral: no documentation, specialised for homogeneous cases, ensures communication, widely used across codebase.
- PhysIntegral: documented, not specialised and does not ensure communication, scarcely used.
To simplify things, PhysIntegral should be removed and integral substituted making sure no bugs are introduced.