re #9014: Bring Reducer and ReductionSingleton to SANS folder
re #9014: Bring reduction steps to sans_reduction_steps

While moving, I've tried to remove those methods and attributes that we
were not using at all. It might have more, which must be investigated in
another opportunity.
re #9014. Bring Instrument to isis_instrument
re #9014: Addapt isis_reducer to the changes

Do not use more things on reduction folder, but instead the SANS ones.
re #9014 Update centre_finder to not user old sans_reduction_steps
re #9014. Adapt ISISCommandInterface to use only SANS folder
re #9014 Improve usage of mantidplot when not in GUI mode

While testing the changes, I've seen that we were silently ignoring
exception when trying to use mantidplot in script mode. It is better to
check if it is available and use only when it is.
re #9014. Improve the solution for long log

The changes here were introduced in #8949, but while testing the changes
through the systemtests I reckognise that some times it may return a
single value. Hence, here it tests first if there are more than one value
before selecting the last one.
re #9014. Ensure assumption of receiving workspace reference

While checking this ticket I found that some system tests were passing
string to this method and were 'silently' being ignored.
This, fixes it.
re #9014: Move the reducer_singleton to scripts folder

This is to allow Indirect to use the same module. And because it is not
strictly related to SANS.
re #9014: Move ReductionStep to reducer_singleton module

After discussion, this allows Indirect to use all they need to implement #9093.
Merge remote-tracking branch 'origin/feature/9014_isis_sans_dependency'