simmer 4.0.0

• The simmer license has been changed to GPL >= 2.

New features:

• The C++ core has been refactorised into a header-only library under inst/include (#147 closing #145). Therefore, from now on it is possible to extend the C++ API from another package by listing simmer under the LinkingTo field in the DESCRIPTION file.
• New generic monitor constructor enables the development of new monitoring backends in other packages (179f656, as part of #147).
• New simulation-scoped logging levels. The log_ activity has a new argument level which determines whether the message is printed depending on a global log_level defined in the simmer constructor (#152).
• set_attribute and set_global gain a new argument to automatically initialise new attributes (#157). Useful to update counters and indexes in a single line, without initialisation boilerplate.

Minor changes and fixes:

• Enhanced exception handling, with more informative error messages (#148).
• Refactorisation of the printing methods and associated code (#149).
• Allow empty trajectories in sources and activities with sub-trajectories (#151 closing #150).
• Enable RCPP_PROTECTED_EVAL (Rcpp >= 0.12.17.3), which provides fast evaluation of R expressions by leveraging the new stack unwinding protection API (R >= 3.5.0).
• Replace backspace usage in vector's ostream method (2b2f43e).
• Fix namespace clashes with rlang and purrr (#154).