Normalize the string -> obj pairs

[777arc]

eg START_INDEX_KEY should probably be renamed to CORE_SAMPLE_START, along with all the others that clearly dont match

[Teque5]

Yep, this should be done along with #92 and throw a deprecation warning when accessing the old key.

[gmabey]

I would add that such variable-value pairs should be generated automatically (albeit through a manually-invoked process) with some python foo.
Same for the extensions (after they are converted to JSON schema).

[777arc]

Yeah good idea, as long as there are generated docs to go with it. E.g. colon -> underscore and capitalize everything.

[777arc]

could also use SAMPLE_START instead of CORE_SAMPLE_START

[Teque5]

@777arc I have created a branch feature/key-simplification that closes this and #92. Please inspect it and try it out. It will emit a DeprecationWarning once when each key is accessed the old way (sigmf.SigMFFile.SOME_KEY, or when a deprecated name for a key is accessed (HASH_KEY vs SHA512_KEY):

In [8]: sigmf.SigMFFile.COMMENT_KEY
<ipython-input-8-370b3c32672b>:1: DeprecationWarning: SigMFFile.COMMENT_KEY is deprecated and will be removed in a future version, use sigmf.COMMENT_KEY
  sigmf.SigMFFile.COMMENT_KEY
Out[8]: 'core:comment'
In [9]: sigmf.SigMFFile.COMMENT_KEY
Out[9]: 'core:comment'

Now I think I'm happy with it, but I had three options for renaming keys:

1. SAMPLE_RATE_KEY
2. SAMPLE_RATE (no suffix)
3. CORE_SAMPLE_RATE

I did a survey of many other python modules and having the _KEY suffix is not common but it was our prior convention. So right now I have implemented option 1 but I could trivially go with option 2 depending on your feedback. Remember keys are stored in keys.py and imported to top level so option 2 would be that you would access the key via sigmf.SAMPLE_RATE. Is that better without the suffix?

Here are the keys that were renamed (but still currently accessable):

Old Name	New Name	Value
START_INDEX_KEY	SAMPLE_START_KEY	core:sample_start
LENGTH_INDEX_KEY	SAMPLE_COUNT_KEY	core:sample_count
START_OFFSET_KEY	OFFSET_KEY	core:offset
HASH_KEY	SHA512_KEY	core:sha512
FLO_KEY	FREQ_LOWER_EDGE_KEY	core:freq_lower_edge
FHI_KEY	FREQ_UPPER_EDGE_KEY	core:freq_upper_edge
LAT_KEY	LATITUDE_KEY	core:latitude
LON_KEY	LONGITUDE_KEY	core:longitude

If we proceed I'll create a new issue that removes the deprecated methods after least one year is elapsed.

[777arc]

Yep that all sounds reasonable!