Ensure that IO is described as an integral graph subsystem

[knoepfel]

Per the CDR reviewers' comment:

Additionally, directly discussing the consequences of Phlex’s design on how it handles I/O would benefit from additional explanation. Having separate providers and persisters for each data product has consequences that users may find surprising. Additionally, although the I/O handling is delegated to FORM, which to our understanding is developed in very close coordination with Phlex, the I/O is inherently an integral part of the framework. This is also demonstrated by several of the requirements being I/O related.