function accessors

[kylebarron]

Change list

• Pass the current RecordBatch into the data object so that it can be received by the user's function callback
• Wrap the user's function callback with our wrapAccessorFunction so that we can transparently handle mapping the "exploded" table from multi-geometry input back to the accurate original row.
• Updated example for both point and multilinestring
• Updated docs

Closes #98