Initial commit for flat_map and flat_multimap

[mintel1]

Initial discussion point for flat_map and flat_multimap. Would like to find out best test practice, plus how to include an actual implementation (from cppreference) as opposed to a boost include

[Twon]

There should be tests added here for these containers, but it make me realise that this test still need to be extended to support all associative containers (i.e. map, unorder_maps, etc):

Morpheus/libraries/core/tests/serialisation/adapters/std/ranges.tests.cpp

Line 27 in d33ad93

TEMPLATE_TEST_CASE("Verify serialisation of sequence containers std::ranges", "[morpheus.serialisation.ranges.serialise.sequence_containers]",

Perhaps this should be deferring to Catch2's generator framework here: https://github.com/catchorg/Catch2/blob/devel/docs/generators.md

[mintel1]

Will take a look at the Catch2 Generator framework thanks.
I haven't used any of the Catch frameworks before but have heard good things

[Twon]

The conformance header should have a simple test to exercise and demonstrate the abstraction over the std and boost types work uniformly, see an example with Date here:

Morpheus/libraries/core/tests/conformance/date.tests.cpp

Line 8 in d33ad93

TEST_CASE("Ensure date library is supported and working", "[morpheus.conformance.date]")