New features causal modeling with twins, and many smaller improvements!

umx 4.15.0

• NEW: umx_merge_randomized_columns wide-2-mingled (rather than wide2long) to unify data from people assigned to different conditions.
• NEW: umxDiscTwin Discordant twin test for causal effects.
• NEW: umxDiscTwin Graphical output for publication.
• NEW: umxPlot ease customized/journal-close, e.g. y~x, -> x y plot, with r^2.
• IMPROVED: umxAPA t-test reporting.
• IMPROVED: umx_score_scale gains error and proportion.
• IMPROVED: umx_score_scale report only omega total when 1 factor extracted.
• IMPROVED: umx_read_prolific_demog gains functionality.
• IMPROVED: umxMR: gained `tryHard'
• IMPROVED: umxGxEwindow: gained tryHard, smarter print control.
• IMPROVED: umxGxEwindow: removes un-needed columns.
• IMPROVED: umxGxE checks that selDefs are numeric (closes #196, h/t @salvatoj)
• IMPROVED: umxRun: gained summary control.
• IMPROVED: umx_read_prolific_demog more control, better defaults, and more reliable summary table.
• IMPROVED: umx_score_scale cleaner output for 1 factor case with omega.
• FIXED: xmu_twin_print_means now takes >1 covariate h/t @eren Kafadar #201
• FIXED: xmu_safe_run_summary now takes refModels = FALSE h/t @lf-araujo #200
• FIXED: umxACEcov plot and summary working (needed to filter covs from DVs).
• FIXED: xmu_twin_print_means handles more than 1 covariate and improves labeling.
• FIXED: umx_score_scale handles NA correctly.
• SAFE CODING: umxPath stops when v*m* is used with firstAt or fixedAt.
• SAFE CODING: umxAlgebra warns that it isn't functional.
• MINOR: umx_print call fixed where was being called with file.
• HELP: umxCP help .svg figure instead of `.png`` (smaller).
• HELP: umxMR Remove unsupported 'weights' parameter.
• ADMIN: copyright, tests, Rd improvements.