v0.1.0

• switch group and unit definitions, to be consistent with the literature
• add Theil's Information Index (H)
• add entropy function
• add mutual_within function to decompose weighted within indices
• add "wide" option to mutual_local and mutual_within
• add "ipf" (iterative proportional fitting) function and a difference decomposition based on IPF
• "mrc_adjusted" difference decomposition is defined only on overlap sample of units and groups
• internal refactoring