v0.4.0

This is a major new release with some new features, greatly improved ergonomics
for structured formulae, matrices and specs, and a few small breaking changes
(most with backward compatibility shims). All users are encouraged to upgrade.

Breaking changes:

• include_intercept is no longer an argument to FormulaParser.get_terms;
  and is instead an argument of the DefaultFormulaParser constructor. If you
  want to modify the include_intercept behaviour, please use:

  Formula("y ~ x", _parser=DefaultFormulaParser(include_intercept=False))

• Accessing terms via Formula.terms is deprecated since Formula became a
  subclass of Structured[List[Terms]]. You can directly iterate over, and/or
  access nested structure on the Formula instance itself. Formula.terms
  has a deprecated property which will return a reference to itself in order to
  support legacy use-cases. This will be removed in 1.0.0.
• ModelSpec.feature_names and ModelSpec.feature_columns are deprecated in
  favour of ModelSpec.column_names and ModelSpec.column_indices. Deprecated
  properties remain in-place to support legacy use-cases. These will be removed
  in 1.0.0.

New features and enhancements:

• Structured formulae (and their derived matrices and specs) are now mutable.
  Internally Formula has been refactored as a subclass of
  Structured[List[Terms]], and can be incrementally built and modified. The
  matrix and spec outputs now have explicit subclasses of Structured
  (ModelMatrices and ModelSpecs respectively) to expose convenience methods
  that allow these objects to be largely used interchangeably with their
  singular counterparts.
• ModelMatrices and ModelSpecs arenow surfaced as top-level exports of the
  formulaic module.
• Structured (and its subclasses) gained improved integration of nested tuple
  structure, as well as support for flattened iteration, explicit mapping
  output types, and lots of cleanups.
• ModelSpec was made into a dataclass, and gained several new
  properties/methods to support better introspection and mutation of the model
  spec.
• FormulaParser was renamed DefaultFormulaParser, and made a subclass of the
  new formula parser interface FormulaParser. In this process
  include_intercept was removed from the API, and made an instance attribute
  of the default parser implementation.

Bugfixes and cleanups:

• Fixed AST evaluation for large formulae that caused the evaluation to hit the
  recursion limit.
• Fixed sparse categorical encoding when the dataframe index is not the standard
  range index.
• Fixed a bug in the linear constraints parser when more than two constraints
  were specified in a comma-separated string.
• Avoid implicit changing of the sparsity structure of CSC matrices.
• If manually constructed ModelSpecs are provided by the user during
  materialization, they are updated to reflect the output-type chosen by the
  user, as well as whether to ensure full rank/etc.
• Allowed use of older pandas versions. All versions >=1.0.0 are now supported.
• Various linting cleanups as pylint was added to the CI testing.

Documentation:

• Apart from the .materializer submodule, most code now has inline
  documentation and annotations.