FIX/STY Small fixes

Among those:
* Use preload=True when rejecting epochs
Move generate_filtered_epochs to epochs module
Move noise CSD computation out of tf_dics
WIP Improve tf_lcmv API

* Move noise covariance calculation outside of tf_lcmv
* Move data filtering in frequency bands inside tf_lcmv
Small cosmetic changes
Add tstep vs win_length check and test and speed up test_tf_dics
Tiny improvements to tf beamformer examples
ENH Use empty room noise for tf_dics example
ENH Scale computed CSD to allow different time window lengths
ENH Use empty room noise in tf_lcmv example
Use n_fft parameter for CSD computation and in tf_dics
FIX us np.allclose for frequency comparison

Not only does this allow tolerance for numerical issues, but also works for
both np.array and float, whereas (x != y).any() as used previously failed when
x and y were floats.
FIX Add noise_csd scaling to tf_dics test
Refactor data reading into separate function in LCMV tests

This is in preparation for further tests to be written now.
Add tests for _lcmv_source_power
ENH : some clean in tf dics + tf lcmv
Improve comments in tf_lcmv example
FIX Use raw_noise to construct epochs_noise
FIX selecting epochs from epochs_noise
Tests for tf_lcmv