Tangents on curves

[tobydriscoll]

Tangents can be handy and might be required in some circumstances. But they place a burden on user constructed curves that may have no payoff in every case. We should make them optional, and then check if one exists wherever it is about to be used.

[ehkropf]

Agreed.

I propose the base curve class throw an error with message ident CMT:NotDefined in its tangent definition. Classes that need the tangent from a curve should use the try/catch mechanism and check for this error. This would be an example of using the error message idents discussed in #55.