change type of _indexMap in PartitionDescriptor to fix issue with older Eigen versions

elsa/core/PartitionDescriptor.cpp

 #include "PartitionDescriptor.h"
 
-#include <map>
+#include <unordered_map>
 
 namespace elsa
 {
@@ -58,7 +58,7 @@ namespace elsa
             throw std::invalid_argument("PartitionDescriptor: cumulative size of partitioned "
                                         "descriptor does not match size of original descriptor");
 
-        std::map<index_t, std::size_t> sizeToIndex;
+        std::unordered_map<index_t, index_t> sizeToIndex;
         _blockOffsets[0] = 0;
         for (index_t i = 0; i < getNumberOfBlocks(); i++) {
             auto it = sizeToIndex.find(slicesInBlock[i]);
@@ -127,4 +127,4 @@ namespace elsa
         coeffsPerDim[_numberOfDimensions - 1] = numberOfSlices;
         return std::make_unique<DataDescriptor>(coeffsPerDim, getSpacingPerDimension());
     }
-} // namespace elsa
\ No newline at end of file
+} // namespace elsa

elsa/core/PartitionDescriptor.h

 #include "BlockDescriptor.h"
-
 namespace elsa
 {
     /**
@@ -67,7 +66,7 @@ namespace elsa
 
     protected:
         /// maps a block index to the index of the corresponding descriptor in _blockDescriptors
-        Eigen::VectorX<std::size_t> _indexMap;
+        IndexVector_t _indexMap;
 
         /// vector of unique DataDescriptors describing the individual blocks
         std::vector<std::unique_ptr<DataDescriptor>> _blockDescriptors;
@@ -88,4 +87,4 @@ namespace elsa
         /// generates the descriptor of a partition containing numberOfSlices slices
         std::unique_ptr<DataDescriptor> generateDescriptorOfPartition(index_t numberOfSlices) const;
     };
-} // namespace elsa
\ No newline at end of file
+} // namespace elsa