Add some kind of command line control over running just one phase?

[lisad]

I started working on this but then wasn't sure how to do it.

I'm thinking of syntax something like
python3 -m phaser run boston output --phase aggregate-counts sources/bike_ped_counts.csv

or

`python3 -m phaser run boston output --phases [1,2] sources/bike_ped_counts.csv'

It's powerful when working on just one phase, to run just that phase and get to ignore output from other phases. But I'm not sure whether to name a phase and run everything up to that phase, or to name a phase and expect the prior input to already be there; also I think it might be easier to provide numbers so that "--phases [1,2]" means to run the first and 2nd phases of the pipeline (definitely easier to type and remember than the phase name, which is different from the phase class name)

[jeffkole]

MVP: run a single phase, expecting all necessary sources to be in the working directory already

If it is easy to extend to run any phases, then do that.