#659 - Add math and trig functions

This change adds FuncAggregator, which does most of the work for
applying a function to each point in a select query.  To demo this,
three new aggregators have also been added which use the
FuncAggregator:  log10, pow, and tolower.  Example uses:

select log10(val) from myseries

select pow(val, 2) from myseries

select tolower(email) from myseries