Fix Halo constructor for DGVectorHolder

[TomMelt]

Fix Halo constructor for DGVectorHolder

Fixes #1064

Task List

• Linked an issue above that captures the requirements of this PR
• Defined the tests that specify a complete and functioning change
• Implemented the source code change that satisfies the tests
• Commented all code so that it can be understood without additional context
• No new warnings are generated or they are mentioned below
• The documentation has been updated (or an issue has been created to do so)
• Relevant labels (e.g., enhancement, bug) have been applied to this PR
• This change conforms to the conventions described in the README

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Change Description

After simplifying #1005 I accidentally introduced a bug by making the DGVectorHolder constructor directly call the DGVector one i.e.,

nextsimdg/core/src/include/Halo.hpp

Line 59 in 2165b00

template <int N> Halo(DGVectorHolder<N>& dgvh) { Halo(static_cast<DGVector<N>&>(dgvh)); }

This is incorrect and leads to a segmentation fault.

I have now modified the constructor to use the Member Initializer List, which will ensure the object is stored correctly

nextsimdg/core/src/include/Halo.hpp

Lines 60 to 64 in 6f337fb

	template <int N>
	Halo(DGVectorHolder<N>& dgvh)
	: Halo(static_cast<DGVector<N>&>(dgvh))
	{
	}

I added a test to check it works correctly, and I have fixed a small typo in the naming of initializeHaloMetadata (which was intializeHaloMetadata before)

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Test Description

I added MPI_TEST_CASE("DGVectorHolder", 3) to HaloExchangeCB_test.cpp to check that halo exchange works correctly for DGVectorHolder

[joewallwork]

Ah yes, easy mistakes to make. Thanks for the fixes @TomMelt.