Extract abstract ListScheduler class

Extracted an abstract ListScheduler class, seperated abstract and non-abstract scheduler classes and updated scheduler tests with direct form IIR 1 and FFT.

Closes: #280 (closed)

added Scheduling label

requested review from @oscgu95

assigned to @simbj106

changed the description

enabled an automatic merge when all merge checks for 16c8327a pass

approved this merge request

merged