Update netcdf-java and cdm version

We currently use v5.1.0, the current version of netcdf-java is at least 5.5.1 (latest stable release, see https://github.com/Unidata/netcdf-java/releases). The latest release addresses a vulnerability in the logback library. This will also help us stay in sync with planned updates to the netCDF-java library (deprecate old API in favor of new public API, immutable core classes, etc.).

• update netcdf-java version
• lock cdm and netcdf version together
  • remove cdmVersion from gradle.properties
  • edu.ucar:cdm:${cdmVersion} becomes edu.ucar:cdm-core:${netcdfVersion} in dependencies.gradle
  • replace ucar.nc2.dataset.NetcdfDataset with ucar.nc2.dataset.NetcdfDatasets