Add support for Nix file format

[shashwatsridhar]

SWAN currently supports only the Blackrock file format (.nev). To extent compatibility to virtually any data, support for an open file format would be ideal. In this regard, Nix is probably the best way forward, since it is well supported by Neo and is designed to handle the kind of data that SWAN handles.

[mdenker]

Currently, efforts focus on openephys, nix integration will come as a future step once Neo allows writing NIX files for lazy mode.