WIP ENH 5D time-frequency beamformer

This PR aims to implement the 5D time-frequency beamformer as presented in Dalal et al.

Currently using dics_source_power, keeping time window length constant across frequencies and lacking plotting. Very much WIP, but submitting now to try to be verbose, plotting should hopefully come tomorrow.


List of TODOs and ongoing issues:
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UPDATE 16 Sep 2013:

* [x] [1] `tf_lcmv` doesn't yet produce reasonable results, needs investigation
* [ ] [2] using the multitaper mode needs to be investigated further as discussed above, i.e. why is low-frequency activity detected only in high frequency bands when multiple tapers are used?
* [ ] [3] using e.g. a 300 ms baseline for all frequencies leads to weird results
* [x] [4] the `data_csd` vs `noise_csd` frequency equality is still commented out and waiting to be corrected
* [x] [5] there's a bug I just discovered which appears when `tstep = 0.1`, needs investigation
* [x] [6] the plot needs a black bar between 55 and 65 Hz
* [x] [7] there are no tests for `tf_dics` (nor for `tf_lcmv` and `_lcmv_source_power` for that matter)
* [x] [8] the `tf_dics` API is not yet documented
* [ ] [9] the maximum source power isn't found in the auditory cortex

Ad 9:
The maximum source power isn't found in the auditory cortex, but it's found where LCMV finds it, which is reassuring. @mluessi suggested it might be because we're using a surface source space and beamformers are mostly used with volume source spaces.

UPDATE 17 Sep 2013:

* [x] [10] the example runs too slowly and should be speeded up, e.g. by parallelizing or picking a single vertex