Write stream files with a selection of the reflections from another stream file

[fredericpoitevin]

(I know you already are working on it Ariana, just thought I'd put this here for my own records)

We'd like to be able to:

• read a given stream file
• do some analysis on the reflections
• select reflections based on this analysis
• write a stream file with only the reflections selected

[apeck12]

Thanks for creating an issue for this, Fred! Much easier to keep track of to-do's this way.

Note to self on this topic: it may be useful to additionally keep track of the chunk number in the stream_data self variable to simplify generating a new stream file with select reflections. This way, if a chunk doesn't have legitimate reflections, it can be skipped altogether.

Second note to self on this topic: the stream_data variable is a bit unwieldy, so it may be worth adding one-line functions to the StreamInterface class that returns the correct column of stream_data when called.