Spatially varying source term and permeability/conductivity/diffusion const

Problem

Currently the source term and material properties (permeability/conductivity/diffusion const) for the heat equations (fluid and solid) are uniform on the mesh. It would be useful to have the functionality to read in a spatially varying source term.

This is related to #71 because the tissue perfusion model requires spatially varying properties.

It is also related to #77 and #291, which is in turn related to work @zasexton is doing (#364) on spatially varying boundary conditions for #71. (in other words, we have a spatially-varying spaghetti of interrelated issues).

Solution

An implementation similar to the user-defined profile, where a file is read to define the spatially-varying values at each mesh node should work. I had implemented this in svFSI previously. Alternatively, it can be stored as a field on a mesh file.

Additional context

No response

Code of Conduct

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Start by reviewing the existing user-defined profile implementation and the related issues #71, #77, #291, and #364 to understand the competing approaches. The work is complete when source terms and material properties can be supplied as spatially varying values, with the input format and mesh-field approach resolved.