Add support for Volume Source Spaces in simulation code

Setup

I have a Volume source set up, with accompanying forward solution and inverse operator. Trying to make sense of my code, I wanted to make up some simulated raw data by generating a VolSourceEstimate or VolVectorSourceEstimate instance, and applying my forward operator on it.

Problem

After gitter-chat with some nice people, it is clear that the mne.simulation namespace does not contain functions that can work with volume source spaces. Also, mne.forward.apply_forward_raw can neither be applied to VolSourceEstimate (forward non-linear in that case, thus difficult to implement, I guess) nor to VolVectorSourceEstimate (which should be easier to deal with).

Proposed solution

Add support for volume source spaces in the mne.simulation namespace, with similar function as mne.simulation.simulate_stc, to simulate a volume source space.
This would be followed by a function similar to e.g. mne.simulation.simulate_raw, where one could generate the measured EEG/MEG signal induced by the simulated source estimates.

Start by reading mne.simulation.simulate_stc and mne.simulation.simulate_raw, then review mne.forward.apply_forward_raw and the VolSourceEstimate and VolVectorSourceEstimate cases described in the issue. Done means the simulation namespace supports volume source spaces and can generate measured EEG/MEG signals from simulated volume source estimates.