add DAMethod guavaFunctionApplyMethod and implicitMapperMethod properties

guavaFunctionApplyMethod is a refactoring to make identification of Guava's Function interface implementation method more accurate (can still be improved by checking parameter type, soon)
implicitMapperMethod is a new property opening the way to Mapper class not implementing Guava's Function interface
this two new properties are computed by DASourceClass's Builder in build() method
modify DASourceClassValidator to rely on new properties and remove every check on DASourceClass interfaces (Guava's Function interface is not required anymore)
rename DAMethodPredicates#isGuavaFunction() to isApplyWithSingleParam() to be more accurate (and avoid confusion with new DAMethod#guavaFunctionApplyMethod property) + added check on the number of parameters (must be 1)
add DAMethodPredicates#isNotConstructor(), DAMethodPredicates#isGuavaFunctionApply() (based on new DAMethod property)
first unit test for implicite mapper method