Switch to use the upstream source code of visit_struct

[traversaro]

This PR switches to use a commit on the upstream branch of ami-iit's fork of visit_struct. That branch is created by cherry-picking the CMake structured added in ami-iit/visit_struct#1 on the top of the latest commit of the master branch of https://github.com/garbageslam/visit_struct, that got VISITABLE_INIT in https://github.com/garbageslam/visit_struct/pull/24/files .

Using the upstream source code of visit_struct would simplify packaging the library in conda-forge . However, as far as I understood the code is different between https://github.com/garbageslam/visit_struct/pull/24/files and cbeck88/visit_struct#14, so we may want to merge with care. The tests of matio-cpp are passing fine, but I do not know if there is something else we want to check before merging.

[S-Dafarra]

Actually, the modification of cbeck88/visit_struct#14 are useful for the weird initialization of

matio-cpp/test/ExogenousConversionsUnitTest.cpp

Lines 56 to 64 in 12022e5

	struct nestedStruct
	{
	using array_3f = std::array<float,3>;
	BEGIN_VISITABLES(nestedStruct);
	VISITABLE_DIRECT_INIT(array_3f, array, {1.0, 2.0, 3.0});
	VISITABLE(testStruct, s);
	END_VISITABLES;
	int* notSupported = nullptr;
	};

. It is not mentioned in the README either. We started using visit_struct in robometry, so we may want to check it keeps compiling too (although I am not expecting big surprises).