Change calls to `_tfr_from_mt` with support for ndarrays

[tsbinns]

Fixes #285.

_tfr_from_mt now supports ndarray after https://github.com/mne-tools/mne-python/pull/13104. Means we can avoid looping over epochs dimensions when passing in data.

Also fixes a bug from #232 where _tfr_from_mt would be imported even when not required, making everything incompatible with MNE<1.10. Now it is only imported when required.