some bug fixes and modifications of arrival times functions

- Doppler shift of `pulse_period' will be calculated properly
- if orbit parameter is NULL in `check_ephemeris' the return value of the orbit is NULL also
- if orbit parameter is NULL in various functions any orbital effect will not be considered
- qualifier of `plot_atime' to overplot the pulse ephemeris changed to `ploteph'
- new qualifier `plotorb' of `plot_atime' overplots the Doppler shifted constant pulse period
- labels of `plot_atime' corrected
- help of `atime_set_ephemeris' will be found now