Make PyPREP's random channel picks match MATLAB PREP's for the same random seeds

[a-hurst]

PR Description

Closes #61. This PR does two main things:

1. Re-implements MATLAB PREP's custom randsample function so that we can generate identical random channel picks using the same random seed
2. Changes the RANSAC sample size from (0.25 * number of good channels) to (0.25 * number of all channels), matching MATLAB PREP and AutoReject's implementations.

I also adjusted the "too few channels" exceptions to be more informative, based on the second change. Let me know if you have any questions!

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

[a-hurst]

Argh, I keep accidentally making style mistakes. Gotta fix VSCode's linting setup to automatically catch this stuff...

[a-hurst]

@sappelhoff Okay, finally fixed the unit tests! Apparently bad-by-flat channels get removed entirely from the data during NoisyChannels, so I wasn't triggering the 'num_good_chans' < 'ransac_sample_size' like I thought I would. Double-checked and MATLAB PREP does the same thing, so no additional changes needed to match it.

[a-hurst]

Made one additional change to better match MATLAB PREP: instead of rounding (0.25 * number of channels) up, they round it to the nearest integer, so I tweaked PyPREP to do the same. Here's the relevant MatPREP code:

ransacSubset = round(noisyOut.ransacChannelFraction*size(data, 2));

[sappelhoff]

LGTM, thanks :)