align schemas with names from IFCB v2 header files

[joefutrelle]

header files for new-style instruments include a list of ADC columns. It is not expected that the order of ADC columns will change, but possibly new ones will be added. Users may also want to use those column names instead of constant names from the schema classes.

One approach is to use dictlike schema classes keyed by the names in the header files, but this is a problem because the ADC column names could change per-bin, depending on what the instrument manufacturer decides to put in ADC files.

It's possible that bins could inject ADC column names from headers into subclasses of the schema object that they return, but this is a complicated implementation that breaks the idea that schemas can be derived from pids.