Version 1.4.21

[TomSnijders]

Description

This is for version 1.4.21. I incorporated (I think) all changes that were in 1.4.20. That version had trouble today with finalizing the pushrelease. I hope this will go better.
The main new things in this version are new functions selectionTable and influenceTable.
Furthermore, a small but misleading error was =corrected: in siena.table, fixed parameter values are now not reported any more as NA, but as their fixed values.

Checklist:

Checks

• [X ] If possible, I have added tests that prove my fix is effective or that my feature works
• [ X] The package builds on my OS without issues
• [ X] My changes generate no new warnings

Documentation

• [X ] I have commented my code inline, particularly in hard-to-understand areas
• [X ] If function added/modified, I have added/modified documentation (in .R file)
• [X ] If effect added/modified, I have added/modified section 12 of the manual
• [X ] I have added a description of all changes to this pull request above and to the NEWS.md file
• [ X] I have bumped the version in the DESCRIPTION and man/RSiena-package.md files by the appropriate increment:
  • 'major' (incompatible API changes)
  • 'minor' (add functionality in a backwards compatible manner)
  • 'patch' (backwards compatible bug fixes)