Updated unit tests to reflect changes to `run_details.py` and fixed minor error
that tests caught after updating.

Removes chaining callbacks together to access a dictionary. This is one
of the things originally added to the scripts which should have been
removed.

This overly complex piece of code attempted to take the DRY pricnciple
to an extreme. Instead each instrument should just specify the layout of
their file

This reverts commit 83b653be.
The previous commit was put in as a solution for a starter exercise
for the next isis_powder developer - it is not actually part of this
work

This reverts commit 30040c35.

This reverts commit 67938cfc.
I realised that this would be a good exercise for the next person
to take over isis_powder, so have left the commit in for reference

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

This allows them to be renamed more conveniently, a al RenameWorkspaces

When the total current in a run is 0 (ie no counts were received), don't
bother normalising by current (as we get a divide by 0 error), and also
don't bother subracting the empty as it gives negative counts