Add verify=True argument to Curve and Triangle/Surface constructors

[dhermes]

In #156 I realized that it's possible to pass an invalid degree and things don't "break", though they should. Parts of this change:

• verify=True should be opt-out the same way _copy=True
• _copy=True should be promoted to public copy=True (_copy being "internal" is too confusing)

The reason __init__() doesn't do any computation on degree (i.e. the reason the from_nodes() helpers exist) is because computing degree from number of nodes may be expensive (for curves it's just num_nodes - 1 but for triangles / surfaces it requires solving a quadratic) whereas verifying will be less expensive. Also, by making verify a keyword argument, callers have the option to opt-out of the verification.