updates to computer-algebra and numerical-analysis

computer-algebra:
  added more content in "variables and expressions", "arithmetic and logic",
  and "strings" sections.  Removed "regular expressions" section.  Also added
  content to the Mathematica column and bumped version up to 10.0.

numerical-analysis:
  renamed "linear models" section as "regression"
  moved the "optimization" section up per order in skeleton.