ENH: Reorder IO and filtering chapters.

It makes more sense to have the IO chapter first
since the examples use IO for filtering.  The
other trainging materials also follow this pattern
of showing simple IO conversion before modifying
data objects.

Change-Id: I3471608b63986d1954f21fff0bcddefd6ba7b24f
Merge "ENH: Reorder IO and filtering chapters."