Gamma power speed up

"Fix" to #185. See also the discussion in #166.

Two improvements:

 - Speed-up the calculation of x^\gamma, x^{\gamma-1} and x^-(\gamma-1) . Currently support \gamma=5/3, \gamma=4/3 and \gamma=2.
 - Write the details of the hydro scheme in the snapshots.

Branch can be removed.

See merge request !195