Add reciprocal trig functions

[derivitaandrew]

It would be nice to see the addition of the reciprocal trigonometric functions: csc, sec, and cot, as well as their inverses: arccsc, arcsec, and arccot.

This would make formulas easier to enter and results more human-readable. For d(tan(x)), compare current 1/(cos(x)^2) to proposed sec(x)^2.

Also, this would settle the issue of cot(pi/2). Since cot(-pi/2), cot(pi/2), cot(3pi/2), etc all equal 0, we currently have to write custom code to check for this pattern, since the only way to enter cot(pi/2) in Algebrite would be 1/tan(pi/2), which should simplify to 0, but understandably won't, since tan(pi/2) is undefined.