Allow codadefs to reinterpret content of self decribing formats

[svniemeijer]

This issue covers the general solution for #13.

The idea is to allow codadefs to be used to (re)interpret data from HDF4, HDF5 and netcdf formatted files. (for CDF, GRIB, rinex, and sp3, this currently does not seem needed).

This will allow:

• single-element arrays to be treated as scalars (or vice versa)
• addition of special types, such as time values
• addition of conversions (unit conversion and/or missing value mapping)
• potentially interpreting text data using a specific structure (similar to XML)

We should combine this with a global CODA option that will either read products using the dynamic format or using the codadef format. If the dynamic format is used a codacheck will then still be possible and will allow to present all issues instead of stopping at the first issue found (which is what will happen when (re)interpreting the product using the codadef as format).