Write multiple binary files

We add the ability to add multiple binary files with signal data to a Writer object. This is in order to support writing of MFFs with EEG and PNS data. By default, the Writer class will prevent the user from writing an MFF that does not adhere to the EGI format (EEG data contained in signal1.bin, PNS data contained in signal2.bin). This behavior can be overridden by setting BinWriter._compatible to False.

We also fix a bug that prevented adding separate epochs to a BinWriter object with 0 offset between them. Previously, data blocks added with offset_us=0 would be added to the same epoch. Now, if offset_us is any non-negative integer, the data blocks will be placed in separate epochs. If offset_us=None the data blocks will be placed in the same epoch.