step_is_waveform is not general, only corresponds to maccor

[ardunn]

step_is_waveform only works with maccor files.

it can be implemented on a datapath-by-datapath basis so that everything specific to Maccor waveform goes in MaccorDatapath, everything specific to Arbin waveform goes in ArbinDatapath, etc. Like a method step_is_waveform.

The default in BEEPDatapath ABC can be evaluating to false, with a log printed that says waveform is not implemented for this specific cycler.

Right now waveform steps will evaluate to false even if there is waveform (e.g., arbin waveform) and there is no logging or indication the step actually is waveform, it just returns false.