Add pyxid2 device for nirx fnirs recording triggers

[JohnGriffiths]

This PR creates another minimal device type, exactly the same concept and general structure as the serial device type. That is, its only purpose is to push triggers, and does not (unlike e.g. the brainflow devices) also have the more important task of controlling a streamed data recording.

This code was written and added out of necessity, as the pyxid2 library appears to be the only way to send triggers to the NIRX nirsport2 via a USB cable. The serial device type does not work for this.

Example usage:

from eegnb.devices.eeg import EEG  as eegexpy_device
thisdev = eegexpy_device(device="nirsport2", xid_num=1)  
thisdev.push_sample(marker=2, timestamp=0) # note that timestamp is not currently used
                                                                           # in this context
# for the nirx nirsport2 the above can be viewed and checked at the bottom of the aurora 
# recording software

# ( in full context: )
from eegnb.experiments.visual_n170.n170 import VisualN170  
thisdev = eegexpy_device(device="nirsport2", xid_num=1)  
thisexp = VisualN170(eeg=thisdev, duration=300, use_fullscr=False, screen_num=1)  
thisexp.run()