Allow C++ code sharing

[pratikunterwegs]

This pull request fixes #146, and allows the {finalsize} C++ code to be shared with other Rcpp packages, such as {epidemics}. This is a potential precursor to fixing #68, but that issue is not tackled here.

Using {finalsize} headers in a new package is relatively easy:

1. Add 'finalsize', 'Rcpp' and 'RcppEigen' under LinkingTo in the DESCRIPTION,
2. Add // [[Rcpp::depends(finalsize)]] in a source file,
3. Import the {finalsize} header using #include <finalsize.h>.

[pratikunterwegs]

I've forcibly reset the branch to remove the final two commits which had moved the contact matrix processing (for the small speed boost) to the finalsize.R file. This functionality should likely stay within the C++ code for now, so that this speed boost is available to users importing the finalsize namespace in C++.