Use FlatLinearData for storing converter results

Previously we returned ElementLinearData from the converter, then
stripped out the MetaLinearData. This meant that before processing
the ElementLinearData from the converter actually contained metadata
which is confusing.

The new document constructor stores the converter results in a
FlatLinearData object and simultaneously populates element and meta
data stores.

Also in this commit I have moved various methods from ElementLinearData
to FlatLinearData, from which ElementLinearData inherits.

Change-Id: I64561bde2c31d8f703c13ac7b0a0c5f7ade9f3d4
Merge "Use FlatLinearData for storing converter results"