Allow finalsize C++ code sharing

[pratikunterwegs]

This issue is to request that the C++ solvers from {finalsize} should be exportable so that they can be used in other Rcpp-based packages. A template for how to do this can be found at this PR on the {epidemics} package: epiverse-trace/epidemics#3.

This would entail:

1. Creating headers under 'inst/include/' which define a C++ namespace (also called 'finalsize'), which contains the solvers which are moved here;
2. Creating (or auto-creating) a package header under 'inst/include/' that includes all other headers;
3. Writing a wrapper function called ".final_size(...)" under a new source file 'src/finalsize.cpp' that wraps the solvers as well as any previous preparation steps (see Move finalsize specific element from solver function to main finalsize() function #68).
4. Writing a small vignette that shows how to link other Rcpp packages with the header provided in {finalsize}.

This would not introduce any breaking changes.

This issue could work towards resolution of #68.