Add offline keys definitions

display offline file keys definitions and units from aanet version 2 documentation

km3io/definitions/events.csv

-#type,name,comment
-
-int,id,offline event identifier
-int,det_id,detector identifier from DAQ
-int,mc_id,identifier of the MC event (as found in ascii or antcc file)
-int,run_id,DAQ run identifier
-int,mc_run_id,MC run identifier
-int,frame_index,from the raw data
-ULong64_t,trigger_mask,trigger mask from raw data (i.e. the trigger bits)
-ULong64_t,trigger_counter,trigger counter
-unsigned int,overlays,number of overlaying triggered events
-TTimeStamp,t,UTC time of the start of the timeslice the event came from
-vec Hit,hits,list of hits
-vec Trk,trks,list of reconstructed tracks (can be several because of prefits,showers, etc)
-vec double,w,MC: Weights w[0]=w1 & w[1]=w2 &  w[2]]=w3
-vec double,w2list,MC: factors that make up w[1]=w2
-vec double,w3list,MC: atmospheric flux information
-double,mc_t,MC: time of the mc event
-vec Hit,mc_hits,MC: list of MC truth hits
-vec Trk,mc_trks,MC: list of MC truth tracks
-string,comment,user can use this as he/she likes
-int,index,user can use this as he/she likes
-int,flags,user can use this as he/she likes