added PREDICTPERC function
there is one still open question: should we interpolate to get the final value?
Say: if we have 8 values, and we want thd 95th percventile, then
we should actually take the 6.65th 0-based index value (=95/100*(8-1)).
with the current implementation we round, so we return actually the 100th percentile.
what we could also do is:
value=val[floor(idx)]+(idx-floor(idx))*(val[floor(idx)+1]-val[floor(idx)])
(besides some boundry checking for the explicit percentile 100)

this should get decided prior to final merging
added interpolation for percentile
Merge pull request #465 from msperl/master-predictperc

added PREDICTPERC(entil) CDEF function