Added checks for the proton frq for the relax_data.read and relax_data.back_calc user functions.
This is in response to the problems seen by Martin Ballaschk <ballaschk att fmp-berlin dott de> and
reported at http://www.mail-archive.com/relax-users@.../msg01179.html (Message-Id:
<533CDA92-36FE-4E36-8925-7536EFF2608E@...>).
If the frequency is not exact or is less than 1 MHz, warnings are now given to the user.
