Always keep EEG data in Volts instead of converting to mV

[a-hurst]

PR Description

This PR removes a bunch of unnecessary conversions back and forth between Volts and mV, likewise changing the units for a few user-facing EEG attributes (e.g. self.EEG_before_interpolation) in PrepPipeline from the EEGLAB standard of mV to the MNE standard of volts.

This should make the API less confusing for people used to working in the MNE world, avoid a bunch of unnecessary large matrix multiplication, and makes some other proposed API changes in #99 easier to implement.

Merge Checklist

• the PR has been reviewed and all comments are resolved
• all CI checks pass
• (if applicable): the PR description includes the phrase closes #<issue-number> to automatically close an issue
• (if applicable): bug fixes, new features, or API changes are documented in whats_new.rst

[codecov-commenter]

Codecov Report

Merging #102 (8fe116a) into master (7ad292c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
- Coverage   99.04%   99.04%   -0.01%     
==========================================
  Files           7        7              
  Lines         736      734       -2     
==========================================
- Hits          729      727       -2     
  Misses          7        7              

Impacted Files	Coverage Δ
pyprep/prep_pipeline.py	100.00% <100.00%> (ø)
pyprep/reference.py	99.18% <100.00%> (-0.02%)	⬇️

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[sappelhoff]

Thanks @a-hurst!