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Add Nonlinear Solver Section to Heat_Transport_BHE

removing extra blank line

remove tailing white space

add more description

Needs to be imported with:

try:
    import ogs.callbacks as OpenGeoSys
except ModuleNotFoundError:
    import OpenGeoSys

`ogs.callbacks` is needed when imported via pip installed package.
`import OpenGeoSys` else.