Two-mode network effects and positivity constraint corrected in Phase 2

[nynkeniezink]

Description

Effects

• Effects toU and WXX now also available for two-mode networks.
• New effect group dyadThirdBipartiteObjective.

Bug correction

• Adherence to positivity constraints during parameter updates in
  Phase 2 corrected for the doubleAveraging=TRUE option.

Documentation

• Explanation of effect groups dyadBipartiteObjective,
  dyadSecondBipartiteObjective, and dyadThirdBipartiteObjective,
  involving a dyadic covariate and a dependent two-mode network, added.

Checklist:

Checks

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

Documentation

• I have commented my code inline, particularly in hard-to-understand areas
• If function added/modified, I have added/modified documentation (in .R file)
• If effect added/modified, I have added/modified section 12 of the manual
• I have added a description of all changes to this pull request above and to the NEWS.md file
• 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)