Update / convert values

Both update and convert can be used to change column values in a DataFrame.

Difference between these operations:

• convert allows changing the type of the column, update doesn't

• parse is a special case of convert to convert String into different types automatically based on its contents

• unfold is a special case of convert to convert DataColumn of objects into ColumnGroup based on properties of objects.

• update allows filtering cells to be updated, convert doesn't

• fill is a special case of update to replace missing values