FilterWithBuffer: make get_sample non-virtual

virtual added to apply() and reset() declaration inline with base class.
This should have no impact on memory usage or functionality
DerivativeFilter: update and slope non-virtual

No classes inherit from DerivativeFilter so no need to be virtual.
Saves 4bytes of RAM.
LowPassFilter: make methods non-virtual

No classes inherit from this class so no need for set_cutoff_frequency,
set_time_constant and reset(T) to be declared virtual.  Saves 6 bytes of
RAM.