add ability to set plugin search path programatically

[jswhit]

Unidata/netcdf-c#3034 adds the ability to set the search path for the compression plugins programatically. This will be included in version 4.9.3 of the C lib. We should add this to the python API. This will allow users to install the plugins separately from the python package and the C lib, and then use the python interface to tell the C lib where to find them.