Cleanup mlcontext (schema handling, converters, meta data)

This task tracks various minor issues that we should cleanup before the 0.11 release. In detail, this includes:

1) Frame column name handling
2) Remove redundant and inefficient converter functions (e.g., for text outputs)
3) Cleanup block sizes, which are sometimes set to 1 for unknown matrix characteristics, which has severe performance implications for subsequent matrix operations.