remove dead comment/code
give normal_lines a numerical option

Also, all solutions have to be checked since if None is
returned in checksol for a test of assumptions, e.g. that
a solution is real, then that solution will be returned.
We don't want that here so we check the numerical value
but keep the symbolic one (unless prec is given).
raise Line error only in LinearEntity
do simplification only when necessary in Point
use dydx to get slope

The slope formula is based on the given point and the point on the
ellipse; but if a point is on the ellipse then that formula cannot
be used to compute the slope: the dydx form should be used.