Consider removing plot by default

[Neves-P]

Quoting @richelbilderbeek #90:

plot_sims = TRUE,

This argument should be removed: it is needlessly 'convenient', and complicates
the code needlessly instead. In the doc, use DAISIE_plot_sims in the
example code and redirect the user using @seealso to DAISIE_plot_sims