Access self.index instead of self._index where possible

[mroeschke]

Description

Since index is defined as

@property
def index(self):
    return self._index

Get and set to self.index when possible. Setting to self.index ensures that we may not be creating an invalid IndexedFrame with a len(index) != len(columns).

There are times when still setting self._index was necessary because some data was being swapped "inplace" and validation needed to be avoided. (Hoping to avoid this pattern in the future)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

/merge