gff3-filter throws exception when field can't be converted to int or float

[mamarjan]

For example, when running with "field feature == 5", there is an exception. Need to decide if this should be an error, a warning, and how to handle this, using exceptions or something else.

Another example, which makes it all more complicated, is this: "(attr some_number - 5) == 0". Currently in the code I would have to return some default number for "some_number", or use an exception and catch it somewhere.

[mamarjan]

The solution using exceptions would kill performance. For floats, NaN could be used, but there is no equivalent for integers.

[mamarjan]

Now when a field cannot be converted to a boolean (i.e. it's not "true" or "false"), or an integer, or floating point number, the filter returns false and the record doesn't pass the filter.