Tweaks to plotting functions

[mikemeredith]

args list now has ask=NULL but same behaviour. traceplot and densityplot do not touch par(mfrow...) when per_plot=1. Only par settings which have been changes are restored.

Ran the example in shinyjags and plots seem to be well-behaved.

[mikemeredith]

Latest commit has several changes:

• Fixed: plotting functions failed with only one chain; replaced loop to plot lines with matplot.
• plot.jagsUI was "ask"ing when number of plots == plots per page.
• densityplot uses same bandwidth for all chains.
• densityplot attempts to detect parameters constrained to be non-negative or in {0, 1} and plots them truncated appropriately.

[kenkellner]

Thanks Mike. Sorry for the delay, the failed tests are due to very small changes in plot appearance that I'll fix soon.