updates to: db, unix-shells, numerical-analysis, computer-algebra, misc-math

db:
  renamed relational databases; removed mongo

unix-shells:
  added assoc array section, filled in some
  fish entries, ksh namespaces, alternatives to variable expansion
  syntax using sed and tr, rm note on zsh bug that is no longer
  reproducable, improved array section

numerical-analysis:
  fixed broken images

computer-algebra:
  work on mathematica and sympy, new sections; added singular column

misc-math:
  new sheet with gnuplot, glpk, pari/gp, coq