Change HAMexternalHeatFlux to mixed (Robin Type) instead of fixedGrad…

…ient (Neumann Type)

- more stable than fixedGradient

For further information:
- https://cpp.openfoam.org/v11/classFoam_1_1mixedFvPatchField.html#details
- https://doc.cfd.direct/notes/cfd-general-principles/mixed-fixed-value-gradient

Update README

Cleanup tabs in the code

Adapt code for of-v9.0

Adapt code for of-v8.0

Bugfix: Correction to commit 854ffdc

-While reverting, correctBoundaryConditions() also evaluate() and change the already reverted boundary values
-updateCoeffs() only updates the coefficients without changing boundary values

Bugfix: Correction to commit 854ffdc

-While reverting, correctBoundaryConditions() also evaluate() and change the already reverted boundary values
-updateCoeffs() only updates the coefficients without changing boundary values