Gamma power speed up

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

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.

Added 1 commit:

• 846c5ac6 - Correct header guards

mentioned in issue #166 (closed)

Added 1 commit:

• 8ba27aa3 - Missed one file in the commit

Added 1 commit:

• c3e24e72 - Do the same for pow(x, gamma-1)

LGTM, handing off to @pdraper.

Reassigned to @pdraper

Added 1 commit:

• fa2b946e - Fix documentation syntax, enable MATHJAX

Needed some documentation fixes, but otherwise looks good.

Status changed to merged

mentioned in commit 489556aa

mentioned in issue #185 (closed)

Thanks. Did not know about the MATHJAX option in Doxygen. Looks much better now.

@nnrw56, it actually leads to a nasty overflow in some situations where x^5 is huge but x^5/3 is reasonable.

I have replaced it with a safer version:

__attribute__((always_inline)) INLINE static float pow_gamma(float x) {

#if defined(HYDRO_GAMMA_5_3)

  const float cbrt = cbrtf(x); /* x^(1/3) */
  return cbrt * cbrt * x;      /* x^(5/3) */
#else
 //...
}