Data-cat

*****
* Decapitalized all references to ``"QD"``.
* Import assertions from AssertionLib_ rather than CAT_.

*****
* Updated the handling of assertions, see ``CAT.assertions.assertion_manager``.

*****
* Lowered Python version requirement from >=3.7 to >=3.6.
* Changed the ``dataCAT.Metamanager()`` class from a dataclass
into a subclass of ``collections.abc.Container()``

*****
* Updated many ``__str__`` and ``__repr__`` methods.
* Added the ``Database.__eq__`` method.
* Moved context managers to ``dataCAT.context_managers``
* Moved (and renamed) the ``DF()`` class to ``dataCAT.df_collection.DFCollection()``.
* Added more tests.

*****
* Introduced a proper logger (see https://github.com/nlesc-nano/CAT/issues/46 and
https://github.com/nlesc-nano/CAT/pull/47).


[Unreleased]
************
* Empty Python project directory structure.


.. _AssertionLib: https://github.com/nlesc-nano/AssertionLib
.. _CAT: https://github.com/nlesc-nano/CAT