Fix SourceChGain/SourceChOffset handling in BCI2k reader

[HansujaB]

Reference issue

Fixes #13851

What does this implement/fix?

• Parse BCI2000 *list parameters correctly by treating the first token as list length.
• Parse numeric values with embedded units via a fn _parse_value_with_unit
• I have removed fn _parse_sampling_rate and used the above fn for that as well
• Apply SourceChOffset and SourceChGain calibration per channel so EEG data is returned in volts.

Tests

• Updated BCI2k reader tests to check real-file header parsing for:
  • SourceChOffset == ["0", "0"]
  • SourceChGain == ["0.1muV", "0.1muV"]
• Wrote a unit test for unit parsing checks for voltage and frequency suffixes.

Questions

• Do we want the unit parser test (test_parse_value_with_unit) ?
• Do we also want a test to check scaling is right?
• Should I edit examples/io/read_bci2k.py as well?