R support

This introduces a number of small changes to make it possible to build scrm's sources with R. Most changes improve scrm's build system or clean up the code; They are useful in other contexts, too. Hence, I would like to include them in the normal branch. It adds

- The possibility to create a model from arguments given as a `std::string`
- Removes all calls to `std::cout` in the code
- Removes unneeded import statements
- Removes some unused variables
- Removes the first_last_tmrca debug summary statistic
- Add a separate header file `macros.h` that stores common macros (e.g. for defining `dout`)

More importantly, the summary statistics system will change a bit to allow the export of summary statistics as data structure rather than as text only. This still needs to be done.