Adds deinterleave

[ferchaure]

Draft to solve #28

Main issues:

• Is it appropriate to add e-focus everywhere in this way?
• The new isx_utils.py file imports NumPy, an ISX dependency, for a single function that could be written in plain Python. It may be worth deciding whether NumPy is a dependency we want to rely on here, or if this should be handled more optionally.
• Multiplane registration must detect different e-focus values and merge them together.
• This draft currently has no tests.