In response to #912, we want to change the way CMOR reports errors for dataset frequency and time axis interval mismatches.

Currently, CMOR will use a default time interval of 0 days if no frequency is defined leading to a time interval error when defining a time axis or writing data to a variable. This error should make it more clear that the dataset attribute frequency was not defined either by the user or the variable table.

Additionally, we would like to reword the current error message for the frequency not matching the time axis interval.

An example of the current error message:

Error: approximate time axis interval is defined as 2592000.000000 seconds (30.000000 days), for value 1 we got a difference of
86400.000000 seconds (1.000000 days), which is 96.666667 % , seems too big, check your values       

Proposed format change:

Error: Dataset was defined with the frequency 'mon', which has an interval of 30.0 days, but the time axis contains an interval of 1.0 days.
Please adjust the time values in your data to be consistent with the selected frequency.

These error messages are generated from the following section in cmor_axes.c.