FIX averaging epochs not recreating times precisely

Fixes beamforming tests mentioned in #1365

`evoked.times` are not recreated precisely in `EvokedArray`. Before it was introduced averaging epochs just copied times, so I brought back that behaviour and this fixes the broken tests because they all relied on time points (e.g. location of maximum in time)
I think this should be:

    self.first = int(round(tmin * info['sfreq']))

Otherwise, e.g. -0.19979521 (200ms) gets later recreated as -39 and then as -0.195 instead of the previous value or, at least -0.2.

This bit of code was recently introduced, but it actually appears in several places, which means there might be a more significant bug. WDYT? Open issue? I'm not sure of the impact.