Ember: adds reading global string definitions from OTF2 traces

[IzajaszPWrosz]

This change extends the OTF2 motif in Ember, so that it can read the global string definitions from an OTF2 trace into memory. String definitions (e.g., function names) are referenced in OTF2 event packets through stringRef ids. This change will allow Ember to interpret these references. There are also local (per rank) definitions in OTF2, but this change is concerned only with reading the global strings.

To avoid issues with potentially lots of string definitions in large trace files, the loading is disabled by default. To enable one has to set the readGlobalStringDefinitions config parameter:
...
ep = EmberMPIJob(0, topo.getNumNodes())
...
ep.addMotif("OTF2 tracePrefix=trace.otf2 readGlobalStringDefinitions=True")
....

[sst-autotester]

Status Flag 'Pre-Test Inspection' - - This Pull Request Requires Inspection... The code must be inspected by a member of the Team before Testing/Merging
NO INSPECTION HAS BEEN PERFORMED ON THIS PULL REQUEST! - This PR must be inspected by setting label 'AT: PRE-TEST INSPECTED'.