Release 0.3.0

Changes

New features

• We can create our own encodings such as Treatment or Sum encoding. These are subclasses of Encoding.
• Added two aliases T and S that are shorthands of C(var, Treatment) and C(var, Sum) respectively.
• DesignVector, CommonEffectsMatrix and GroupEffectsMatrix now retrieve their values when passed to np.array() and np.asarray().
• Add poly() stateful transform.
• na_action in design_matrices() can be "pass" which means not to drop or raise error about missing values. It just keeps them (#69)
• design_matrices() gains a new argument extra_namespace. It allows us to pass a dictionary of transformations that are made available in the environment where the formula is evaluated (#70)

Maintenance and fixes

• Fixed a bug in the addition of lower order terms when the original higher order term wasn't full-rank.
• Columns for categorical terms are integer by default. They are casted to float only if there are other float-valued columns in the design matrix.
• Updated str and repr methods of ResponseVector, CommonEffectsMatrix, and GroupEffectsMatrix.
• Added str and repr methods for DesignMatrices.
• Added get_item method for DesignMatrices.
• Added support for comparison operators within function calls.