validate input file format in dxt_analyzer
This MR makes dxt_analyzer do the following on invalid format:
carns@carns-x1-7g ~> ./dxt_analyzer.py -i foo.darshan Error: unable to parse foo.darshan. Please make sure that it was generated by the darshan-dxt-parser utility.
Example of correct behavior:
carns@carns-x1-7g ~ > ./darshan-dxt-parser foo.darshan > foo.dxt carns@carns-x1-7g ~> ./dxt_analyzer.py -i foo.dxt
It confirms a) that the file begins with a line of text and b) that the line includes the string "# darshan"
Fixes #286 (closed)