isIP Function

  function isIP()
    !+ Check whether the system in use is IP or SI
    logical :: isIP
    if (PSYCHROLIB_UNITS == IP) then
      isIP = .true.
    else if (PSYCHROLIB_UNITS == SI) then
      isIP = .false.
    else
      error stop "The system of units has not been defined."
    end if
  end function isIP