Static-frame

Improved implementation of ``Frame.pivot``.

Removed class-level documentation injection, permitting better static analysis.

Corrected issue in appending tuples to an empty ``IndexGO``.

Added ``Frame.from_clipboard()`` and ``Frame.to_clipboard()``.

Added ``Frame.pivot_stack()`` and ``Frame.pivot_unstack()``.

Fixed flaw in difference operations on ``IndexDatetime`` subclasses of equivalent indices.

Frame.from_parquet`` and ``Frame.from_arrow`` now accept a ``dtypes`` argument.

All ``PathLike`` path objects now accepted wherever ``Path`` objects were previously.

Added ``fillna`` methods to ``Index``, ``IndexHierarchy``.

Added to ``StoreFilter`` the following parameters: ``value_format_float_positional``, ``value_format_float_scientific``, ``value_format_complex_positional``, ``value_format_complex_scientific``.

``Index`` and ``IndexHierarchy`` will reuse instances for set operations on equivalent indices.

Added ``IndexHierarchy.from_names`` constructor for creating zero-length ``IndexHierarchy``.

Refinements to ``IndexHierarchy`` to support grow-only mutation from zero length.

Fixed flaw in ``IndexLevel`` for handling of zero-length levels.

Fixed flaw in ``TypeBlocks.iloc`` that caused an undesirable reference cycle.