fix bug with masking conditions when asking for bounds on quantiles

e.g. "q_prd_median{>quantile0.3,<quantile0.7}" was not working
because the string was too long, but the regex were also quite
complicated, so this revises the approach and simplifies the regex

but to overcome the length limitation with C++ regex, "quantile" now
need to be specified as "qtl" to be shorter