`mmif-utils` package

[keighrim]

mmif-utils python module should provide python APIs for common operations that are useful for different apps.

[keighrim]

First feature we want to add to the module can be translation btw timeunits (frame numbers <--> seconds, given fps of the source)

[keighrim]

Another component that could be helpful is file (media) I/O (more I, though). Related to #152, we'd like to add more ways to encode files locations in somewhere else than a local filesystem, and hence, it's good to have a common code to read files from say, HTTP, s3, etc.

[keighrim]

This issue now is split into and superseded by #222 and #224.