Mutation operator for methods returning classes with a default constructor

[oscarlvp]

Create a mutation operator that generates code for methods that return an instance of a class having a default constructor (with no parameters).

[oscarlvp]

This may create equivalent mutants in many cases. If implemented it shouldn't be active by default.

[oscarlvp]

This is being partially implemented in pull request #94
The implementation in the pull request concerns only classes which are in the classpath of the PIT execution.
A difference could be to also allow classes which are in the project and not necessarily in the classpath of PIT.

[oscarlvp]

#94 was merged with 956a55c
The issue remains open to check if we can:

• obtain information from the classpath of the project and check if these classes have constructors without parameters
• detect methods that return an instance of a class by invoking the default constructor, these can become stop methods for this operator