rewrite hyperbolics unconditionally; inverse defined only for monotonics

inverse should be defined only if there is a single inverse for a given
value. sin(x) = 1/2 has 2 values per period and thus does not have an
inverse defined; tan(x) has a single value per period and has an inverse
defined. This attribute is used in the _invert function to algebraically
isolate the variable of interest. Functions with more than one value should
be handled in _tsolve or elsewhere.
add atan2 solving

The formula for the equivalent formula for atan2(y, x) is from

http://stackoverflow.com/questions/6411484/
is-there-a-way-to-reverse-the-effect-of-atan2
pass along flags from _solve and _tsolve
plotting typo fixed
watch for -Add exponents in factor_terms

If the exponent of a power is -Add, gcd_terms will rewrite it
distributing the -1 because of how Terms and Factors works, so
watch out for that.
use targeted expansion if u is a power
watch for PolynomialError when using _tsolve
wip-mask-nonx
watch for bad simplification