Static-frame

Corrected more issues when calling ``IndexHierarchy.loc[]`` with another ``IndexHierarchy``, or when calling ``Frame.assign.apply`` when that frame has ``IndexHierarchy`` columns.

Corrected undesirable type coercion from happening in single-row selections from ``IndexHierarchy``.

Corrected issue when calling `IndexDatetime.loc[]` with an empty list.

Corrected issue when calling `IndexHierarchy.loc[]` with another `IndexHierarchy`

API change: ``Bus`` no longer accepts a ``Series`` on initialization; use ``Bus.from_series()``.

API change: ``Batch`` no longer normalizes containers after each step in processing; use ``Batch.via_container`` to force elements or arrays to ``Frame`` or ``Series``.

API change: ``Index`` objects can no longer be created with ``np.datetime64`` arrays; such labels must use an ``IndexDatetime`` subclass instead. If this is happening implicitly with an operation, that operation should expose a parameter for ``index_constructor`` or ``index_constructors``.

API change: ``IndexAutoFactory`` is no longer accepted as an ``index_constructor`` argument in ``Series.from_pandas()`` and ``Frame.from_pandas()``; ``IndexAutoFactory`` should be passed as an ``index`` or ``columns`` argument instead.

Minimum Python version is now 3.7

New implementation of ``IndexHierarchy``, offering significantly improved performance and removal of the requirement of tree hierarchies.

Added ``Batch.to_series()``.

Fixed issue when using ``Frame.from_npz`` with an NPZ created with a ``FrameGO``.

Fixed issue when supplying overspecified mappings to ``Frame.astype``.

Further improved handling of binary equality operators with ``IndexDatetime`` subclasses.

Improved handling of binary equality operators with ``IndexDatetime`` subclasses.

Extended interface of ``Batch`` to include all methods for handling missing values, as well as all ``via_*`` interfaces.

Silenced all NumPy warnings where the issue raised in the warning is being explicitly handled in the code.