[DOC] document interpolation of sensor variables...

[jklymak]

Both the dbdreader code and the parquet seaexplorer code linearly interpolate onto a timebase. We should document this in some detail now that we have a variety of different deployment setups. Probably a bit of a project to do comprehensively, but I think we should maybe have a utility that creates a report where the user just needs to specify a cast number (to avoid shallow/bad casts) and it makes some plots of raw data and what we give them. Then we can compile some of the examples. This appendix to the documentation could also include what settings on the gliders were used to get the data set so that we understand what settings set the sampling frequencies and the results are reproducible.