Fix/wrong expid for block

In original design, operator class does not know the correct exp id for each block. The current version of BwdTransStdMat access the wrong expansion in the expansionlist.

The new solution is

• Move GetBlockAttributes() to Field.hpp, so that both Operator and main can access it;
• GetBlockAttributes() does not modify the original sequence of exp in explist. In this design, we may create more blocks of the same element type. So std::map is no longer used;
• Use 3 basiskey to identify different element type, in order to avoid cases when total number of points are the same but basis are different;
• Fetch correct exp_id of each block by offseting the exp_id, starting from 0.

Add new test file square2.xml which contains two different element order.

Operators/BwdTrans/BwdTransStdMat.hpp

@@ -20,10 +20,11 @@ public:
         auto const *inptr = in.GetStorage().GetCPUPtr();
         auto *outptr      = out.GetStorage().GetCPUPtr();
 
+        size_t exp_idx = 0;
         for (size_t block_idx = 0; block_idx < in.GetBlocks().size();
              ++block_idx)
         {
-            auto const expPtr = this->m_expansionList->GetExp(block_idx);
+            auto const expPtr = this->m_expansionList->GetExp(exp_idx);
 
             Nektar::StdRegions::StdMatrixKey key(
                 StdRegions::eBwdTrans, expPtr->DetShapeType(), *expPtr);
@@ -38,6 +39,7 @@ public:
 
             inptr += block.block_size;
             outptr += expPtr->GetTotPoints() * block.num_elements;
+            exp_idx += block.num_elements;
         }
     }