[ENH] Multiple improvements to spectral_connectivity_time

[ruuskas]

PR Description

This PR addresses improvements mentioned in #104.

• Add ciPLV metric.
• Compute all connectivity measures at once, avoiding the repeated computation of the cross-spectral density.
• Reinstitute the option to specify frequencies of interest also when using the multitaper method. The reasoning is that users should have control over the sparsity of the frequency grid.
• Add the option to use parts of each epoch as padding. This allows for the mitigation of edge effects in the time-frequency decomposition. The edges are removed after time-frequency decomposition, but before connectivity computation.
• Improve the function description to better describe the objective.

Unfortunately the Git log for this PR is a huge mess. The reason is that this was started before breaking changes were made in #104. Should probably have rebased instead of merging from main.

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [ ] whats_new.rst is updated
• [ ] PR description includes phrase "closes <#issue-number>"

[ruuskas]

I decided to close this PR and repost with the relevant commits properly cherry-picked on top of upstream/main.