Refs #20477

Refs #20477

Change back AbInitio to DFT (add comaptibility with older naming currently present in stable Mantid).

- Update the renamed CalculateResolution algorithm to NRCalculateSlitResolution
- Fix an incorrectly named variable where theta was called two_theta
- NRCalculateSlitResolution no longer returns two_theta if it was found from the logs, so get it directly from the logs instead. Note that the default log value that was being used for two_theta in CalculateResolution was Theta, which is the same log value we're using here, so there is no change in behaviour.
- Fix a bug where theta was being passed as two_theta to NRCalculateSlitResolution. This will change the result in line with other fixes to NRCalculateSlitResolution in Re #18442.

Re #20529

Python 3 by default, when using str() prints more decimal places than Python 2. This was causing differences in the data after completing ConvertUnits algorithm.

Refs #20440

Also added an assert to see why a test fails. Re #19341

Refs #20477

Refs #20477

Refs #20447

Refs #20477

- Added .DataProcessor where required in python scripts and
documentation.
- Fixed import in data_processor_gui.

- Renamed to ProcessingAlgorithm
- Also renamed ProcessingAlgorithmBase.