Added regved3d for visualizing regression models

matlib 0.7.0

• use corner() in vectors3d.Rd
• added arc() for 2D, 3D vector diagrams to show angles between vectors
• added regvec3d() for 2D, 3D vector diagrams representing a bivariate multiple regression model, lm(y ~ x1 + x2) [thx: John Fox]
• added internal .arrows() to produce nice arrows in 2D vector() diagrams
• fixed numerous small problems in vector diagrams
• fixed some erroneous statements in vignettes
• showEqn() now aligns terms vertically and prints without quotes
• reversed sense of absolute in points_on_line() and clarified documentation