DTU mean sea surface correction

Add feature to use a DTU 1 minute mean sea surface a reference elevation for the ALS L1B elevation.

Updates of note:

1. code now fully uses the processor settings (additional json file)
2. the use of a mss elevation correction must be specified in the processor settings file
3. only two options for elevation references: 1. WGS84 (as before) & 2. DTU MSS netCDF files (preferable 1 minute grid)
4. A new variable (elevation_reference) has been added with a dedicated class for computation for each echo (dependent on time, lon, lat). For WGS84, the class only returns 0.0. For DTU MSS, the class returns a linearly inpolated MSS at the location of each echo.
5. The .alsbin2 file format has changed by adding the elevation_reference variable.

In addition, this merge request contains a few minor code and logging improvements