Based on the automatic case generation in CFDOF, I believe the temperature gradient might actually just be the heat flux itself (in W/m^2), and perhaps there's some rescaling in OpenFOAM, although I'm not sure how this works

Added a notebook showing the calculation of the temperature gradient for OpenFOAM, and updated default values of the temperature field and the output value based on the updated scoping calculations

Removed data_file, thermochimica_path, etc arguments from most functions, to allow more succint function calls, and instead exported them to a dedicated Config class so that these are class variables configured with an instance of tta, but they can of course be updated on the fly if they need to be changed, this is in line with intended/anticipated use.