CRAN release v9.1.0

CHANGES OR IMPROVEMENTS

• Added a new built-in Susceptible-Infected-Susceptible model: SIS.

• Added the functions update_u0 and update_v0 to update the
  initial state in a model.

• Added the pfilter function to run a bootstrap particle algorithm
  on a model. See the documentation for an example.

BUG FIXES

• Fixed the unnamed non-character argument 'usr' in par() when ploting
  the density of the ABC posterior distribution.