Add config class

[J08nY]

Some things in pyecsca should be configurable at runtime such as:

• Whether some error states are detected and raise an exception. For example when computing a modular square root of a non-residue, or when computing a modular inverse of a non-invertible element. This is useful for simulating the behavior of implementations which contain these kinds of errors.
• Whether formula assumptions are verified.
• Maybe what Mod implementation is used? GMPY vs Raw.
• ...