Bump to version 2.0.0

Version 2.0.0 will mark our shift to semver 2.0.0:

> Given a version number MAJOR.MINOR.PATCH, increment the:
> 
>  1. MAJOR version when you make incompatible API changes
>  2. MINOR version when you add functionality in a backward compatible manner
>  3. PATCH version when you make backward compatible bug fixes
> 
> Additional labels for pre-release and build metadata are available as extensions to the MAJOR.MINOR.PATCH format.

DESCRIPTION

@@ -1,6 +1,6 @@
 Package: malariasimulation
 Title: An individual based model for malaria
-Version: 1.6.1
+Version: 2.0.0
 Authors@R: c(
   person(
     given = "Giovanni",