bug #877, bug #572: Get rid of Index conversion warnings, summary of changes:
- Introduce a global typedef Eigen::Index making Eigen::DenseIndex and AnyExpr<>::Index deprecated (default is std::ptrdiff_t).
- Eigen::Index is used throughout the API to represent indices, offsets, and sizes.
- Classes storing an array of indices uses the type StorageIndex to store them. This is a template parameter of the class. Default is int.
- Methods that *explicitly* set or return an element of such an array take or return a StorageIndex type. In all other cases, the Index type is used.
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