mpindex 0.2.1

• Changed returned value of define_mpi_specs from list to data frame with a class of mpi_specs_df. By default, it stores a global option named mpi_specs which can be accessed using getOption('mpi_specs'). Other specifications are now stored as attributes.
• Changed default value of .unit_of_analysis from households to NULL.
• save_mpi now allows unformatted output (tidy format) when generating an output.
• Implemented feature to include MPI specification in the output by setting .include_specs tot TRUE in save_mpi.
• Deprecated .names_separator argument in define_mpi_specs.
• Enhancement of define_deprivation based on #17.
• Added use_global_mpi_specs() wrapper function to use Global MPI specification.