Improve efficiency of Masker process function

[lauramurgatroyd]

Description

The Masker processor needs refactoring to improve efficiency.

The way that Masker performs averaging and interpolating needs improving.

The averaging or interpolating is along a single axis only.

The processor starts by looping over the axis to be averaged along:

CIL/Wrappers/Python/cil/processors/Masker.py

Line 211 in b10d15d

for i in range(arr.shape[axis_index]):

Then it updates any Masked values at the first index on that axes before looping over the remaining indices and repeating.

However, there isn't necessarily a masked value at each index on the axis. It would make more sense for the loop to be over the values in the mask.

Environment

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