The FRD class name is non-pythonic (or at least non PEP8) and generates problems in compiling the documentation on MacOS (see PR #297). If we change the name of the class to FrequencyResponseData this will be more consistent with the other classes (StateSpace, TransferFunction) and also remove this problem on MacOS.

Note that the frd function would still exist for creating frequency response data objects (like ss and tf).