Renamed all source files/programs to more intuitive and recognisable names

* Renamed all source files/programs to more intuitive and recognisable names:
  * all programs now start with gw_ to make it easier to recognise/remember them
  * made program names a bit more descriptive
Added new programs gw_coalescence_pmax, gw_coalescence_time, gw_final_period

* Added new programs gw_coalescence_pmax, gw_coalescence_time, gw_final_period:
  * gw_coalescence_pmax:  maximum orbital P that can converge in a given time
  * gw_coalescence_time:  time needed to reach a ceratain Porb (e.g. 0) due to GWs
    * replaces gw_binary_contact which did the special case for Pf = 0
  * gw_final_period:      orbital P of a binary after a given time of GW evolution
Cleaning up; syncing program name with file name; using syntax_quit() to report syntax
Added source file GWtool.f90 for documentation