updates from Michal Táborský
The source field for guides now understands:
recognizes these patterns or keywords:
any number -> percentile (same as before)
avg, average -> obvious
min, minimum -> 0th percentile
max, maximum -> 100th percentile
med, median -> 50th percentile
stddev -> standard deviation
stddev+ -> average + standard deviation
stddev- -> average - standard deviation
refs #323