Make the name of gstime() public

It was unwarranted for me to mark `gstime()` private, because it’s one
of the routines officially documented in the paper on SGP4, and so
should be available in Python under exactly the same name.  This helps
with the software project described in pull request #31.

sgp4/propagation.py

@@ -1948,7 +1948,7 @@ def sgp4(satrec, tsince, whichconst=None):
 * --------------------------------------------------------------------------- */
 """
 
-def _gstime(jdut1):
+def gstime(jdut1):
 
      tut1 = (jdut1 - 2451545.0) / 36525.0;
      temp = -6.2e-6* tut1 * tut1 * tut1 + 0.093104 * tut1 * tut1 + \
@@ -1961,6 +1961,10 @@ def _gstime(jdut1):
 
      return temp;
 
+# The routine was originally marked private, so make it available under
+# the old name for compatibility:
+_gstime = gstime
+
 """
 /* -----------------------------------------------------------------------------
 *