mention "multiple_filters"-related API changes in the next release notes

[belevtsoff]

The API changes are: 1) There's no f_filter argument in GenericSource anymore 2) All filtering stuff is moved from core.extract.py to core.filters.py

[belevtsoff]

@btel @cpcloud Alternatively, we could make core.extract.Filter still available but with a deprecation warning. I'm not sure if it's worth doing though, taking into account the current number of SpikeSort's active users. What do you think?

Also, taking into account future renaming feature, the detect_spikes's and consequently SpikeDetector's filter arguments are probably not needed anymore. So again, should they be deprecated or just dropped?

[cpcloud]

It depends on whether the filter parameter is frequently used. I personally haven't ever used it. If the thinking is to put it in a separate module altogether I vote for dropping the arguments in detect_spikes and SpikeDetector.

[btel]

yes. you can drop the filter parameter without deprecation warning. just make sure that all tests and example scripts work.

[btel]

duplicates #56