Eliminate Module warnings from tests.

[timspainNERSC]

Eliminate Module warnings from tests.

Fixes #356 (kind of)

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Change Description

Declare Modularized classes as extern to eliminate the last warnings during compilation (on my computer, at least).

Compilation warnings such as

In file included from /Users/timpai/nextsim/dg/nextsimdg/physics/test/ERA5Atm_test.cpp:15:
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:56:9: warning: instantiation of variable 'Module::Module<Nextsim::IFluxCalculation>::spf' required here, but no definition is available [-Wundefined-var-template]
        spf = generator;
        ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/ERA5Atm_test.cpp:62:39: note: in instantiation of member function 'Module::Module<Nextsim::IFluxCalculation>::setExternalImplementation' requested here
    Module::Module<IFluxCalculation>::setExternalImplementation(
                                      ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:101:15: note: forward declaration of template entity is here
    static fn spf;
              ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:56:9: note: add an explicit instantiation declaration to suppress this warning if 'Module::Module<Nextsim::IFluxCalculation>::spf' is explicitly instantiated in another translation unit
        spf = generator;
        ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:57:9: warning: instantiation of variable 'Module::Module<Nextsim::IFluxCalculation>::staticInstance' required here, but no definition is available [-Wundefined-var-template]
        staticInstance = std::move(spf());
        ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:102:31: note: forward declaration of template entity is here
    static std::unique_ptr<I> staticInstance;
                              ^
/Users/timpai/nextsim/dg/nextsimdg/physics/test/../../core/src/include/Module.hpp:57:9: note: add an explicit instantiation declaration to suppress this warning if 'Module::Module<Nextsim::IFluxCalculation>::staticInstance' is explicitly instantiated in another translation unit
        staticInstance = std::move(spf());
        ^
2 warnings generated.

can be solved by adding

extern template class Module::Module<ModularizedClassName>

just before the test source code.