ImageData's .get_slice should allow you to specify 'centre' on any axis

[lauramurgatroyd]

Description

Currently only allows you to specify 'centre' on vertical axis

E.g. myFISTANN.solution.get_slice(horizontal_x='centre')
throws error
ValueError: invalid literal for int() with base 10: 'centre'

Environment

import cil, sys
print(cil.version.version, cil.version.commit_hash, sys.version, sys.platform)