Added effects and turned off interactions for continuous behavior

[nynkeniezink]

Description

• New effects varAlt and avSimVarAlt added.
• Bug correction: Interaction effects for continuous behavior now turned
  off. The R side of includeInteraction for continuous behavior had been
  implemented but not the C++ side, so now an error message is given.
• Error message: If includeInteraction() is called for a continuous behavior
  effect, an error message is given that interaction effects are not yet
  implemented for continuous behavior.
• Coding: For the new effects, lvariance, the behavior variance over all but
  the last wave, was included as member data of BehaviorLongitudinalData
  together with the corresponding member functions.

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)

[TomSnijders]

I gave specific comments on some of the changed files.

[nynkeniezink]

I replied to your specific comments on some of the changed files.

[TomSnijders]

I now have seen all. There is one remaining question, about the sqrt version of the new outindegreebalance effect.

[TomSnijders]

Wonderful.