Add filters to a stream object

[mscheltienne]

Closes #105

Remaining TODOs:

• Add container for IIR Stream filters
• Add creation of a new filter to the base Stream object
• Add filtering and initial condition estimation within the acquisition loop
• Write tests for Stream.filter(...)
• Add notch filtering

[mscheltienne]

@larsoner To complete the short discussion on the issue, could you have a quick look through this PR, especially through the 2 comments I left?
I'm not convince the initial condition zi stay valid if multiple filters are applied one by one to the same channel. Maybe I need to write some code to combine the filters applied to the same channel and thus prevent any overlap in the picks associated with each filter.

[mscheltienne]

After a good night of sleep, I'm actually convince that the initial conditions are not valid anymore. I don't see an easy way to set dynamically the zi for each second order filter in the sos output in _acquire, so instead I will ensures filters do not apply to the same channels or combined their sos arrays if they do, outside of the online-loop.