sdp.py: begin implementing bertini code path
  TODO: parse output from bertini
README: update to reflect inclusion of sympy

Signed-off-by: Jacob Emmert-Aronson <jre2718@berkeley.edu>
sdp.py: finish implementing bertini code path and set as default
    improve documentation

Signed-off-by: Jacob Emmert-Aronson <jre2718@berkeley.edu>
sdp.py: suppress bertini output by default

Signed-off-by: Jacob Emmert-Aronson <jre2718@berkeley.edu>