Salinity harps, CTD & Tsticks

Added routines for dealing with files from the salinity harp, the CTDs and the Tsticks.
read_ctd works with either an output file from the RBR or SeaTerm software or with a log file retrieved from the MicroCat Seabird CTD via minicom.
For the salinity harps there exists a xarray accessor seaice providing the method calc_ice_properties, which computes the quantities brine salinity, bulk salinity, solid fraction and liquid fraction based on the values measured by the salinity harp (i.e. temperature and resistance).