Description of work:
Anna requested that due to new requirements, the fit in y-space routine needs to be run for both backward and forward routines, but with different inputs. Easiest way to achieve this is to split the y-space inputs into the two classes of analysis inputs for both forward and backward.

To test:
Create and sctivate a test environment with mantidworkbench installed:

• Checkout this branch and install with pip install -e .
• Run mvesuvio config
• Open Mantid workbench and open file ~/.mvesuvio/analysis_inputs.py
• In the class BackwardAnalysisInputs change the field run_this_scattering_type = True and in ForwardAnalysisInputs change run_this_scattering_type = False
• Click Run and check that the script runs without errors
• In the class BackwardAnalysisInputs change the field run_this_scattering_type = False and in ForwardAnalysisInputs change run_this_scattering_type = True
• Click Run and check that the script runs without errors

Fixes #xxxx.