homework for new helpers modules

[ambarb]

Some minor work is needed.

Address bits of code that are less likely to fail in testing so far but could fail if conditions are met (flow control choices - most common seem to work ok)

As per @hyperrealist in #89

• find_possible_darks() will run into an unhandled exception at this line if header.stop["exit_status"] == "abort" since exp_time will not be defined in that case.
• Perhaps expected behavior, but find_possible_darks() may return a pandas DataFrame here or a python dictionary here depending on the execution path.

And then also:

• find and remove unnecessary np.copy() from fccd.py and overscan.py
• address if stop document does not get written to database - see comment below.

Far future for a new issue after this one is addressed:

• Optimize further with tiled (should be faster searching and descriptors checking)
• overscan.py optimization after testing is a seperate issue.

[ambarb]

another thing to add to the list is how to handle if the stop document DOES NOT EXIST. This can happen if someone tries to ^C a bluesky session AFTER RE.abort() or RE.stop(). Currently, the ^Z and killing the process is the only way to recover, and in that case, the stop document is never written to data store.
bluesky/bluesky#1626