Refactor `pspm_interpolate`

[dominikbach]

Feature Description

1. Remove option to work on multiple data files at a time.
2. Remove option to work on several channels at a time: either work on all continuous channels (and write new file) or work on one channel only (and write to same file).
3. Use pspm_load_channel for single channel option, and pspm_select_channels for all channel option.
4. Change channel selector in GUI from many to any.