* add appending utility

* add appending utility

* update comment

* rename to mutable

* move mutable functions out of DataFrameHelper

* move mutable functions out of DataFrameHelper

* move mutable functions out of DataFrameHelper

* turn DataFrameHelper into FieldCache

* guess time from name

* graph the numbers

* return the timeField, not just the index

* just warn on duplicate field names

* only use a parser if the input is a string

* append init all fields to the same length

* typo

* only parse string if value is a string

* DataFrame: test fixes

* Switch to null for missing values

* Fixed tests