Frame-wise compression

[arahlin]

Useful for allowing fseek on compressed frame files.

[nwhitehorn]

Is there a reason we want to use fseek? The way we encode pipeline state, and so the whole processing model, kind of break if we do that. For one-off debugging, this is somewhat useful, but you still need some kind of index of where you should fseek to, which means you need to read the whole file once anyway, so just decompressing it seems easy enough...

[arahlin]

The so3g tools use this functionality. We added indexing with fseek/ ftell to the G3Reader already in a previous PR. This open issue is just closing the loop, so that this functionality works on compressed data as well.