Make Engine name optional

[yhql]

Reason

This PR is only a suggestion, based on a perhaps personal view: giving a name to an engine feels unwarranted in most straightforward cases. The changes implemented here make it possible to name an Engine only when needed, and otherwise pick a default name with no user intervention: cpa = CpaEngine(selection_function, guess_range) would now be accepted.

Details

For example, the default CpaEngine would be named "cpa". Several of them would be named "cpa", "cpa1", "cpa2", .... Two CpaEngines and two NicvEngine in the same session would be named "cpa", "cpa1", "nicv", "nicv3" as this PR appends the current number of present engines in the session to their default names. This could be confusing, but is also a case where one would prefer naming their engines manually anyway.

Breaking change

This introduces a breaking change in the Engine's prototype:

- CpaEngine("cpa0", selection_function, guess_range)
+ CpaEngine(selection_function, guess_range, name="cpa0")

As the name parameter is now optional, it is a keyword argument and is placed at the end of the parameter list. Therefore the first parameter changes from the name to selection_function, which will make existing scripts fail with a type error.