v0.8.4

This is a minor update that improves the ability to handle missing values.

A new method is introduced for changing the fill value of missing values, set_fill.

Another method missing_as is introduced. This will be the opposite of as_missing. It will set missing values to a constant value.

Dataset contents will now show the fill value for variables. Furthermore, open_data will now check if the fill value is zero, which can cause problems for logical comparisons etc.