netcdf import and nc viewer

 added controls to select time period and interpolation
 single long fetch is removed.
 USGS Thredds catalog fetch mimics that of browser and catalog tree nodes are expanded on user command.
 This saves a very large amount of time since each fetch takes a couple of seconds and there is no point in 
 creating local xml files to avoid parsing. However .nc files may need caching.
 fixed matplotlib issues when plotting inisde PyQt.
 added progressbar to monitor matplotlib's plotting.

matplotlib operations are not multithreaded.