Fixed incorrect datatype conversion on multi-indexes

Fixed test to use assertTrue

Simplifying tests to write and read back

Simplified multiindex test to use a roundtrip

Release notes update
Merge pull request #8022 from artemyk/sql-create-table-index-dtype-fix

Fixed incorrect datatype conversion on multi-indexes
BUG: fix HDFStore iterator to handle a where properly (GH8014)
Merge pull request #8029 from jreback/hdf_iterator

BUG: fix HDFStore iterator to handle a where properly (GH8014)
BUG/ENH: fix ewmvar/cov debiasing factors; add 'adjust' to all ewm*()