Allow passing units out from onda_samples_to_edf_signals

[ericphanson]

in https://github.com/beacon-biosignals/OndaEDF.jl/blob/fc8709d84e2a55b77cd86d1f9fbc3003d5a95c6d/src/export_edf.jl#L189C1-L189C1

we call

OndaEDF.jl/src/standards.jl

Lines 40 to 42 in fc8709d

	function onda_to_edf_unit(onda_sample_unit::String, unit_alternatives=STANDARD_UNITS)
	units = get(unit_alternatives, onda_sample_unit, missing)
	return lift(first, units)

but don't allow hte user to choose the 2nd argument, so only standard units are supported.

We should allow passing custom units through onda_samples_to_edf_signals

[kleinschmidt]

The current workaround for this is to modify the STANDARD_UNITS dict.