[GIO] New Implementation of IOGEN

[fathollahzadeh]

This PR is a new implementation of the GIO (generating readers for custom datasets). In the new implementation, removed all hard-coded implementations for flat datasets and replaced them with code gen. One of the primary goals of GIO is to support single and multi-row representations of tuples in source datasets. This PR is for supporting both of them. An outline of the PR is:

It supports Matrix and Frame It supports nested and flat datasets It supports non-standard datasets, i.e., an incomplete number of cols in a CSV file