Add Transform to Subtract Images

[anamatoso]

Hello all,

I've created a transform to subtract two images.
I've created the tests and passed all integration tests.
Hope you find it useful.

Let me know if there is anything else still to do.

Description

Adds a transform for subtracting images.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[ericspod]

Hi @anamatoso thanks for the contribution. This class is probably something very specific to your use case so I'm not sure about how general purpose it is. We don't have transforms for doing other arithmetic operations, so I wonder if it would be better defining a generic transform which collects selected keys together and applies some callable to those items. This could be used as a new base for ConcatItemsd as well. What do you think? The class as-is seems fine to me if you'd rather just stick to committing it.