Added documentation for setup functions overriding the filter order

Usually the order is only specified when instantiating the filter which both reserves the memory and then calculates the corresponding coefficients. This is the standard approach.

However for changes of the order during runtime one can specify the max order in the template parameter which then reserves space and specify the actual order later in setup(). To prevent a segmentation fault the setup command now checks if the actual order is equal or less to the instantiated order via the template parameter.