Added census datasets.

Removed warnings.  Updated documentation around multi-stage transfer.
Moved from ivt to vt
Reworked 'value' to 'info' in glyphset.

Removed 'add' from glyphset interface.
Tests for glyphset converter (part of the generalized 'info' implementation).
Made the generic-type named 'I' for the info value in many places where it is used.