SI Units support

[jbfaden]

Chris showed me that the C Das2 library supports SI units, where it knows "1 meter" divided by "1 second" is "1 meter/second". I've got code in the Das2Datum library that does this, and I should make sure it considers what Chris is supporting in the C library. For example, he has a parser and I should support strings formatted and parsed with that code. See https://github.com/das-developers/das2C/blob/master/test/TestUnits.c

[cpiker]

It a tangent, but a companion issue has been opened in das2C to add support for offset units: das-developers/das2C#19 .

[cpiker]

The units module in das2C was originally written to support the das2_psd utility. I wanted the units on the resulting spectrograms to be correct without human intervention but didn't want special case code in das2_psd, thus a generalized units module was written.

The code in units.c consists of six sections:

1. SI Prefix handling, both as symbols and names, i.e: micro and μ
2. SI Unit handling, both as symbols and names, i.e.: ohms and Ω
3. Mathematical operations: i.e. div( "meters", "second") => "m s**-1"
4. Reduction, i.e. "m/s" * "s" -> "m"
5. Conversion: ratio = convert(1.0, "meters/sec", "km/day")
6. Singleton assignment

The fundamental assumption on all the parsing code is that units strings are encoded as UTF-8, period. There is no handling of any other encodings.