Cooling infrastructure

See discussion in email:

• Adopted a common infrastructure for cooling.
• Moved the "constant cooling" to the new infrastructure.
• Created the files for "Creasey cooling". Still need fully working implementation.

doc/Doxyfile.in

@@ -760,8 +760,10 @@ WARN_LOGFILE           =
 # Note: If this tag is empty the current directory is searched.
 
 INPUT                  =  @top_srcdir@ @top_srcdir@/src @top_srcdir@/tests @top_srcdir@/examples
-INPUT		       += @top_srcdir@/src/hydro/Minimal @top_srcdir@/src/gravity/Default
-INPUT		       += @top_srcdir@/src/riemann 
+INPUT		       += @top_srcdir@/src/hydro/Minimal
+INPUT		       += @top_srcdir@/src/gravity/Default
+INPUT		       += @top_srcdir@/src/riemann
+INPUT		       += @top_srcdir@/src/cooling/const
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding. Doxygen uses