Add support for n-dimensional arrays in _tfr_from_mt

[tsbinns]

Describe the new feature or enhancement

As discussed with @larsoner here, #12910 added the function _tfr_from_mt, however it does not work for data with an epochs dim, something required in MNE-Connectivity.

Describe your proposed implementation

Increase flexibility of _tfr_from_mt to allow working with epoched data, similar to _psd_from_mt.

Describe possible alternatives

N/A

Additional context

No response