Merge Rcpp sim code into develop

[thijsjanzen]

• added flag 'use_rcpp' to:

  • DAISIE_sim_core_cr
    and all the upstream functions calling it, e.g.:

    - DAISIE_sim_cr_IW
    - DAISIE_sim_cr_GW
    - DAISIE_sim_cr_CS
    - DAISIE_sim_min_type2
    - DAISIE_sim_relaxed_rate
    
    and the upstream functions calling those:
      - DAISIE_sim_cr
    

Please note that the Rcpp code includes code for different types of islands, but that in DAISIE_sim_core_cr by default all island types are switched off (island ontogeny = 0, sea_level = 0)