Fixed a Multitap linking bug. Added freq() to Multitap.

When declaring Multitap as a pointer (i.e. Multitap<> *delay), an error was thrown during compilation. adding "this->" before mIpol() fixes it. I've also added in a freq() method for individual taps in Multitap.
Fixed Multitap.freq() argument to be consistent with Delay.
Merge pull request #25 from mhetrick/patch-1

Fixed a Multitap linking bug. Added freq() to Multitap.