Add reliability calculations for content analysis

[strohne]

Should be implemented in all effect functions that support a grouping column (effect_counts_one_grouped(), effect_counts_items_grouped(), effect_metrics_items_grouped(), effect_metrics_one_grouped()). The grouping column should be used for holding the coder name (or abbreviation).

Implement reliability as an option for the method-parameter which overrides Cramer's V calculation and Eta-squared calculation. Necessary metrics:

• Holsti / Percent agreement
• Kappa
• Krippendorff's Alpha
• Gwet's AC

[strohne]

Implemented agree_tab() for categorical variables. Can be called on its own or by setting agree to "reliability" or "classification" in report_counts().