zrecast should spot if "pressure" (or other field) is requested twice

[emillour]

When zrecast is run to output in altitude, it automatically generates a "pressure" variable. This can conflict with the user requesting that same "pressure" variable in the list of variables to process (when the problem arises, one gets the error message "Error, could not define variable pressure. NetCDF: String match to name in use"). This should be checked by zrecast.

In fact it might also be helpful that zrecast checks the list of variables to process checking that none is requested more than once.