support subframes

Both Python and Java implementations of FrameRange/FrameSet assume that all frames are whole numbers - ints are used throughout. Possibly other places in the code make this assumption as well.

Ensure that subframes are supported and fix as necessary.

Start by reading the Python and Java implementations of FrameRange and FrameSet, then trace other locations where frame values are assumed to be integers. Map the affected paths before deciding the scope; the work is done when subframes are supported consistently across both implementations and the related frame-handling code.

Written by the indexing model from the issue text.