Remove some MATLAB-isms (kk instead of k, etc.)
Remove a completely unnecessary pre-allocation
Rename "design_func" -> "lowpass_design_func"
Remove the "order" argument

Remove the "order" argument from both the low-pass filter design function and
the class RMFilterBank.  This simplifies things and allows for even more
arbitrary filter design functions (ones that don't know their order
beforehand).
Minor code simplification