WIP ENH dics_source_power and dealing with multiple frequencies

Here's a first implementation of computing source power using DICS. It's not very well tested yet - I'll get down to that once I have the other code to be implemented in this PR and then can easily compare dics, dics_epochs and dics_source_power (and mabye lcmv) on SPM face data.

Things left to do in this PR:

* [x] compute_csd should support returning a list of CSDs,
* [x] dics_source_power should be able to take a list of CSDs and turn it into an stc with frequency instead of time.

Things left to do outside (?) of this PR:

* refactor dics, lcvm, dics_source_power so that there's less code duplication.

I don't think that's crucial, but I would definitely like to do it before the summer is over.