Added support of non-default cdfMod in test script since scipy seems to bee broken
added calls to rm to leave a clean file system behind after running the test suite
 * Added named constants `CDF_MOD_*` for available cdf mods
 * import dataset class of cdf mod to Cdo().cdf instead of whole package
 * loadCdf() is called on first access to Cdo().cdf to initialize it.