introducing the src/data/pulsar collection:

- new function 'pulseperiod2phase'
- rewrote 'slope_lc' and renamed it to 'pulse2pulse_flux_lc'
- rewrote, renamed, and put into a separate file (pulseperiod.sl):
  'pulse_period' to 'pulseperiod'
  'pulse_transform' to 'pulseperiod_transform'
  'check_ephemeris' to 'check_pulseperiod_orbit_struct'
- updated arrivaltimes.sl accordingly to the above changes
- moved pulseprofile_energy_map.sl into this collection
- fixed the help of 'taylor'