allow lazy preproc

[robintibor]

Only load data if preprocessor needs it

[robintibor]

one main use case here is that for mne crop function, otherwise all data is loaded before cropping, clearly undesirable

[agramfort]

there is something easier. You can check the preload attribute of the instance.

if not raw_or_epochs.preload:
     raw_or_epochs.load_data()

[robintibor]

Maybe I should make the comment clearer:
Some mne functions do not need the data to be loaded (e.g., mne crop), and in this case, we want to apply the function without loading the data. I am not aware of any way to know beforehand if the function will need the data to be loaded, hence the try/except.
Before what happened was:

• I wanted to crop out only a small part of the recording using mne crop, and then apply other preprocessings next
• Before mne crop was applied, in our previous Braindecode code (the code changed here), the whole data was loaded unnecessarily
• This makes it substantially slower than in the code now proposed (I did perf-comparisons)

Will clarify the comment in the code