Reads the DDI document on a file system and converts it into an R object. Right now the following information is added to the object:
1. Variables info as well as values labels for categorical variables

ReadDDI(ddiPath, ddiFile)

## Arguments

ddiPath A string containing the path to the directory that has the DDI document A string containing the name of the DDI document

## Value

A named list which is an instance of a BLLFlowDDI class. The list contains the following members:
1. variableMetaData - A named list. It's value comes from calling the
library(bllflow)
pbcDDI <- bllflow::ReadDDI(system.file("extdata", "", package="bllflow"), "pbcDDI.xml")