Feature: Allow pybv to scale to several units (accomodate different channel types)

[sappelhoff]

Currently pybv only accepts a single unit as a parameter:

pybv/pybv/io.py

Lines 62 to 65 in 76e8bea

	unit : str | None
	The unit of the exported data. This can be one of 'V', 'mV', 'µV' (or
	equivalently 'uV') , 'nV' or None. If None, a suitable unit based on
	the selected resolution is chosen automatically.

and that unit is applied to all channels:

pybv/pybv/io.py

Lines 276 to 281 in 76e8bea

	for i in range(nchan):
	resolution, unit = _optimize_channel_unit(resolutions[i], units[i])
	s = r'Ch{}={},,{:0.{precision}f},{}'
	print(s.format(i + 1, ch_names[i], resolution, unit,
	precision=max(0, int(np.log10(1 / resolution)))),
	file=fout)

It would be nice to be able to pass a n_channels long vector of units to pybv, to accommodate different channel types.

For example an auxilliary channel in degrees Celsius, which would be in the unit C.

Note that channel-specific resolutions are already possible, and the proposed feature here is simply a logical extension of that behavior:

pybv/pybv/io.py

Lines 57 to 61 in 76e8bea

	resolution : float | ndarray
	The resolution **in volts** in which you'd like the data to be stored.
	By default, this will be 1e-7, or 0.1 µV. This number controls the
	amount of round-trip resolution. This can be either a single float for
	all channels or an array with n_channels elements.