Optimization for reconstruction variance

Achieved by replacing SVD-based pseudo-inverse with
an eigen solver pseudo inverse.

New function works in the same way: it decomposes
matrix to V*D*V^-1, then inverts diagonal of D
and composes matrix back.

The same way is used to deal with gauges - last
7 eigen values are getting zeroed.

In own tests gives approx 2x boost, without
visible affect on selected keyframe quality.
Keyframe selection code cleanup

- Updated comments in code.
- Removed currently unused functions.
  Actually, they'd better be moved to some generic
  logging module, but we don't have it now so was
  lazy to create one now. Could happen later using
  code from git history
- Renamed function to match better to what's going
  on in it.