Remove external dependency on nctoolbox

[kakearney]

This issue is part of the JOSS review (openjournals/joss-reviews#2497)

This toolbox often relies on the java-based nctoolbox functions to read netCDF files, rather than using Matlab's native netCDF utilities (ncread, ncinfo, netcdf class methods, etc.). The extra external dependency will be a barrier to use for some novice users. Java dependencies also add an additional area for potential incompatiblity.