Derive ARCFileReader from G3Reader

[arahlin]

This ensures that the ARC file reader has the same arguments and functionality as the core G3Reader, and illustrates more generally how to create derived classes for creating streams of frames from other on-disk data formats.

The G3Reader class is refactored so that all of the logic surrounding opening file streams and counting frames is internal to the class, and a FillFrame function is exposed to derived classes to read the stream object and return a populated frame. The required file extension (modulo compression) is an optional keyword argument that is not exposed to python.
The stream object, current file name and frame counter variables are also visible to derived classes.

The ARCFileReader derived class then just configures the file extension in the base class constructor, and overrides the FillFrame function to parse arc data from the steam to populate GCPSlow frames, taking care to parse header information each time a new file is encountered in the stream.

[nwhitehorn]

I'm not sure I understand why this is wanted. It seems like it mixes a lot of things together in order to make a header private.

[arahlin]

I'm not sure I understand why this is wanted. It seems like it mixes a lot of things together in order to make a header private.

The G3Reader has a lot of options that aren't exposed for the ARCFileReader, and this is mainly fixing that. The header doesn't have to be private if you prefer it to remain part of the public API. I made the header private because the code there has been in flux and this would limit the number of other libraries that any code changes there would touch, but maybe that's a moot point with #189

[nwhitehorn]

I don't really like the mixing of G3Reader (which reads .g3 files) with an entirely different type of file reader. It seems like it might make things more fragile in the end by trying to serve too many goals with G3Reader. I would personally prefer to revisit this after #189.

[arahlin]

FWIW there is nothing in the G3Reader code that enforces file extensions or anything to indicate that it's specifically for G3 files. Perhaps that's something to add.

[nwhitehorn]

I'm still really not sure I understand the purpose of this. The point of having all the g3stream stuff is to abstract away the important details of G3Reader so that they can be shared with classes reading things that aren't .g3 files (like the ARC reader). Having the ARC reader inherit from a class that does something else entirely in order to use its wrappers around an abstraction seems architecturally kind of questionable to me. I feel like there has to be something I am missing here...