v0.3.0

This is a relatively massive release bringing lots of new features and bugfixes. All users are encouraged to upgrade. The full changelog can be viewed in the documentation website. The highlights include:

Breaking changes:

• The minimum supported version of Python is now 3.7 (up from 3.6).
• Moved transform implementations from formulaic.materializers.transforms to
  the top-level formulaic.transforms module, and ported all existing
  transforms to output FactorValues types rather than dictionaries.
  FactorValues is an object proxy that allows output types like
  pandas.DataFrames to be used as they normally would, with some additional
  metadata for formulaic accessible via the __formulaic_metadata__
  attribute. This makes non-formula direct usage of these transforms much more
  pleasant.
• ~ is no longer a generic formula separator, and can only be used once in a
  formula. Please use the newly added | operator to separate a formula into
  multiple parts.

New features and enhancements:

• Added support for "structured" formulas, and updated the ~ operator to use
  them. Structured formulas can have named substructures, for example: lhs
  and rhs for the ~ operator. The representation of formulas has been
  updated to show this structure.
• Added support for context-sensitivity during the resolution of operators,
  allowing more flexible operators to be implemented (this is exploited by the
  | operator which splits formulas into multiple parts).
• The formulaic.model_matrix syntactic sugar function now accepts ModelSpec
  and ModelMatrix instances as the "formula" spec, making generation of
  matrices with the same form as previously generated matrices more
  convenient.
• Added the poly transform (compatible with R and patsy).
• numpy is now always available in formulas via np, allowing formulas like
  np.sum(x). For convenience, log, log10, log2, exp, exp10 and
  exp2 are now exposed as transforms independent of user context.
• Pickleability is now guaranteed and tested via unit tests. Failure to pickle
  any formulaic metadata object (such as formulas, model specs, etc) is
  considered a bug.
• The capturing of user context for use in formula materialization has been
  split out into a utility method formulaic.utils.context.capture_context().
  This can be used by libraries that wrap Formulaic to capture the variables
  and/or transforms available in a users' environment where appropriate.