Q7 online

[teuben]

This PR implements the equivalent of the GBTIDL routines "online" and "offline".

1. sdf = GBTOnline()

   Monitors new data in $SDFITS_DATA, $DYSH_DATA/sdfits or /home/sdfits (in that order).
   Any of the "sdf" functions "summary()", "gettp()" etc.
   will check if new data have arrived and refresh the "sdf" object automatically
   (user is informed if that happens).

2. sdf = GBTOnline('foo')

   Monitor a specific project. Possibly an unusual mode, but this mode exists.

3. sdf = GBTOffline('foo')

   Load a specific project, and is thus functionally similar to GBTFITSLoad('foo')

[mpound]

You should add GBTOnline and GBTOffline to dysh/fits/init.py so they can be imported directly from dysh.fits as we do with GBTFITSLoad.
Arguably, dysh/shell/shell.py should also be updated, but I suggest that be made a separate enhancement issue for @tchamberlin after this branch is merged.

[mpound]

I tried it out on gb machines, works as expected except my comments about *args in GBTOnline/Offline, catcher for subbeamnod, and init.py. Fix those and I'll approve.

[teuben]

reminding myself too: can't write the dunder init, but have to write with two backslashes up front __init__ and the rest is history

[mpound]

reminding myself too: can't write the dunder init, but have to write with two backslashes up front init and the rest is history

gets me every time

[mpound]

Nice job! I tried on on gb machine, works as expected.

[teuben]

Need to keep an eye open now for performance (e.g. is there a longer and longer wait) as the file grows