Get test coverage of Curve.py to above 90%

[VikingScientist]

As of 4b98c43

The lines that are not covered by tests are

• 142
• 153
• 159-162
• 193
• 248
• 257-258
• 307
• 367-369
• 371-373
• 438-449
• 452
• 455