Closes #431

This PR sees a lot of refactoring of the io module.

Proposed Changes

• check_multifreq performance has been massively improved.
• A bug that was causing multi-frequency channels in txt files to be exported with the wrong frequency sampling has been fixed.
• breaking API: read_header_and_channels provides a channel list that is already a list of numpy.ndarrays and a full header for acq
• breaking API: extract_header_items requires only the header as input
• breaking API: check_multifreq now returns also the list of channels with the right sampling.

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

Checklist before review

• I added everything I wanted to add to this PR.
• [Code or tests only] I wrote/updated the necessary docstrings.
• [Code or tests only] I ran and passed tests locally.
• [Documentation only] I built the docs locally.
• My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
• My code respects the adopted style, especially linting conventions.
• The title of this PR is explanatory on its own, enough to be understood as part of a changelog.
• I added or indicated the right labels.

• I added information regarding the timeline of completion for this PR.

• Please, comment on my PR while it's a draft and give me feedback on the development!