Read CMIP6 data #1751

Is your feature request related to a problem? Please describe.
Enable pyaerocom to read CMIP6 data directly

Example file name:
od550aer_AERday_MPI-ESM-1-2-HAM_historical_r1i1p1f1_gn_185001-201412.nc
meaning:
<variable name>_<source type>_<source ID>_<experiment>_<realization>_<time start-time end>
Source ID in CMIP terms is the model name in aerocom terms

Example file:
/lustre/storeA/project/aerocom/aerocom-users-database/CMIP6/MPI-ESM-1-2-HAM_historical/download/od550aer_AERday_MPI-ESM-1-2-HAM_historical_r1i1p1f1_gn_185001-201412.nc

Besides the longer possible time frame, the content is pretty much like aerocom:

netcdf od550aer_AERday_MPI-ESM-1-2-HAM_historical_r1i1p1f1_gn_185001-201412 {
dimensions:
	time = UNLIMITED ; // (60265 currently)
	lon = 192 ;
	lat = 96 ;
variables:
	double time(time) ;
		time:standard_name = "time" ;
		time:units = "days since 1850-1-1 00:00:00" ;
		time:calendar = "proleptic_gregorian" ;
		time:axis = "T" ;
	double lon(lon) ;
		lon:standard_name = "longitude" ;
		lon:long_name = "longitude" ;
		lon:units = "degrees_east" ;
		lon:axis = "X" ;
	double lat(lat) ;
		lat:standard_name = "latitude" ;
		lat:long_name = "latitude" ;
		lat:units = "degrees_north" ;
		lat:axis = "Y" ;
	float od550aer(time, lat, lon) ;
		od550aer:long_name = "ambient aerosol optical thickness at 550 nm" ;
		od550aer:units = "1" ;
		od550aer:table = 199 ;
		od550aer:institution = "MPIMET" ;

Describe the solution you would like to see
create a CMIP reader that is usable in the same manner as the EMEP reader

Additional context
Using the EMEP reader as template this looks relatively straight forward