New plotting functions

Some changes to plotting names:

plot_par -> plot_trace
plot_cor -> plot_scatter

Also added:

plot_pairs: scatterplots of all parameters against each other
plot_density: kernel density plots of all parameters

Clean version for training and runiverse

Very small changes from previous version, squashing a few notes that pop up when running checks and also correcting some mistakes in documentation.

This version is penned to be included in the mrc-ide runiverse, which should greatly simplify the installation process.

version 1.5.2

Small changes from previous version, e.g. documentation. Target acceptance rate can now be specified by user.