API: SQL class definitions renaming

- renaming of PandasSQLAlchemy to SQLDatabase and PandasSQLTable to SQLTable
- renaming of SQLDatabase.read_sql to read_query (to be consistent with read_query/read_table)
- legacy -> fallback
BUG: fix Index.reindex to preserve type when target is empty list/ndarray

TST: check index/columns types when doing empty loc/ix tests

CLN: don't _ensure_index in NDFrame._reindex_axes, it is done in
     Index.reindex
SQL/ERR: raise error when insert failed with sqlite fallback (GH8341)
Merge pull request #8462 from immerrr/preserve-index-type-when-reindexing-to-empty

BUG: fix Index.reindex to preserve type when target is empty list/ndarray
COMPAT: disallow Timedelta invalid ops
DOC: v0.15.0 / faq.rst updates
Disallow Series indexing by DataFrame
TST: dtype fixes on tests wrt GH8432
Merge pull request #8472 from rockg/series-indexing

Disallow Series indexing by DataFrame
Merge pull request #8470 from jorisvandenbossche/sql-8314-time

SQL/ERR: raise error when insert failed with sqlite fallback (GH8341)
Merge pull request #8440 from jorisvandenbossche/sql-ooapi-naming

API: SQL class definitions renaming
DOC: fix minor formatting issues
TEST: add for sqlite fallback raising on datetime.time (and not failing silently) GH8341
BUG: .at indexing should allow enlargement scalars w/o regards to the type of index (GH8473)
Merge pull request #8475 from jreback/at_fix

BUG: .at indexing should allow enlargement scalars w/o regards to the type of index (GH8473)
Merge pull request #8474 from jorisvandenbossche/sql-8314-time

TEST: add for sqlite fallback raising on datetime.time (and not failing silently) GH8341
BUG: fix panel fillna ignoring axis parameter

added/fixed tests and re-ordered _is_mixed_type check

fill axis defaults to stat axis

implement inplace; simplify a bit

changed ffill and bfill default axis to None

fixed inplace implementation

added in @staple's tests

return explicitly for inplace

trying Panel.apply

more apply work

merge in new tests

simplify return

update tests with issue num and spacing before comments

added Panel4D.fillna suport and tests

added updates to v0.15.0.txt and updated fillna docstring

added issue numbers to v0.15.0.txt and added note for bug fix

more straightforward fillna method

fix panel4d tests