Getting surfaces of a box monitor

[shashwat-sh]

Added FieldMonitor.surfaces() to monitor.py, and a corresponding test function test_monitor_surfaces_from_volume() in test_components.py.

For an object of class FieldMonitor associated with a box, the method FieldMonitor.surfaces() returns a list of 6 "surface" monitors, each one corresponding to the 6 sides of the box.

Would be useful to review that the functions are written in an efficient "pythonic" way - feedback on style etc. would be great!

[tylerflex]

Needs a few minor changes, but overall looks good!

[tylerflex]

Let's make one more change: I think it will be useful for the FieldTimeMonitor to also have this functionality. Can we move this method to AbstractFieldMonitor and generalize the stuff on line 197? For example, for each of the new surfaces, we can make a copy of self using self.copy(deep=True) and assign the correct centers, sizes, and names to the copies. Let me know if this makes sense.

[tylerflex]

Looks good. Only two comments for things to fix and I'll merge:

• In the docstring, let's replace "box monitor" with "field monitor", since all monitors are technically box monitors and we dont use that terminology in other places.
• Very minor, nitpicking comment: when we are appending the new monitors to the return list, let's initialze the new monitor first (mon_new = self.copy(deep=True)) and then modify it mon_new.size = ... before adding to the list. It's not a big deal, I just think this is a bit cleaner.

Thanks a lot!