axom: Conflicting definition of IndexType

In src/axom/core/Types.hpp we have:

 #if defined(AXOM_USE_64BIT_INDEXTYPE) && !defined(AXOM_NO_INT64_T)
 using IndexType = int64;
 #else
 using IndexType = int32;
 #endif

And in src/axom/sidre/core/SidreTypes.hpp we have:

 typedef SIDRE_IndexType IndexType;

SIDRE_IndexType is elsewhere defined as int64_t. We should define IndexType in only one place.