secr 4.3.1 and other stuff

[murray.efford]

This week -

• secr 4.3.1 (now on CRAN) has some significant bug fixes and a few new features (summarised here).
  
• The first serious release of secrapp (a Shiny app for running secr) is now live (note also the step-by-step guide). The live link lets you run secrapp on a University of Otago machine without installing it yourself. We will see how this works out: peaks in demand may crash it, so for class use etc. you will probably want to run it locally (see readme at GitHub).
  
• Changes in openCR 1.4.2 and 1.4.4 are minor.
  
• The Density web page has been updated: it's time to push the old Windows application Density into the background. Raw input files are provided there for various examples.