Updated several Matlab scripts

[arango]

I continue my slow updating of all the Matlab scripts that use the NetCDF interfaces. As mentioned in ​WikiROMS, we are moving to the native NetCDF interface. However, we still support the MEXNC and SNCTOOLS interfaces.

I added a new function nc_contant.m to return the numeric value associated with the name of a NetCDF library parameter. It is an umbrella on top of the native function or older NetCDF interfaces. For example,

> value = nc_constant('NC_FLOAT')
  or
> value = nc_constant('nc_float')

It turns out that we have several scripts in this repository that use these named string parameters. They are defined if the MEXNC interface is installed... Some of you were having problems when using the the native interface alone without having the MEXNC interface installed.

I also reinstate old function nc_vname.m to get the names of all variables in a NetCDF file. This function is OBSOLETE but it is kept for backward compatibility. Use nc_vnames.m in the future.

I still have more work ahead to port several old scripts to use also the native Matlab interface.