The _format_headers() function in IAGA2002Writer.py would choke when
the sensor_sampling_rate parameter in the trace.stats metadata could
not be inverted. This occurred, at least, when the "Digital Sampling"
IAGA2002 header line was not parsed properly, or was blank.

Erin (Josh) Rigler authored 6 years ago and Claycomb, Abram Earl committed 6 years ago

- allow adjusted and provisional strings in --type controller argument
- allow adjusted, provisional, and definitive strings in _get_url() in TimeseriesFactory
- return appropriate abbreviation for adjusted/provisional data type
- translate adjuste/prvisional to A0 location code in EdgeFactory
- only write out DECBAS for variation/reported data types
- translate adjusted/provisional to A0 location code in IMFJSONWriter.py
- in several places I set it up so that 'variation' and 'reported' were synonymous,
  although I am not sure I found every such instance.

Also update the maximum channel length to 4 in IAGA2002Writer to
allow for the use of 'DIST' as a channel.

Fixes #186
Changing IAGA output value format to follow the standard.

Moved create_empty_channel to Util module. Changed empty channel from - to NUL. Expanded channel definition

Added ability to except 3 character channels, and to automatically pad out to 4 channels,  if less then 4 channels are present.