Compression and cdo flags

[jaseeverett]

We could add a new argument to each function that allows power users to pass additional cdo flags that they want added.

One that is important is compression. My merged files are 10-100 times bigger than the original. Default compression which seems to be the default on many CMIP files is level 1 with backward compatibility to previous nc versions:

cdo_flags <- "-f nc4c -z zip_1 -L"
cdo_cmd <- paste0("cdo ", cdo_flags, " -selname,'", v, "' -mergetime ", 
                  paste(files, collapse = " "), " ", out_file)