[functions.checkerboard] use ConfigTree instead of ParameterTree
[functions.constant] use ConfigTree instead of ParameterTree
[functions.spe10] use ConfigTree instead of ParameterTree
[functions.checkerboard] use GlobalFunctionInterface instead of LocalizableFunctionInterface
[functions] use ConfigTree instead of ParameterTree
[functions. constant] move static methods to the top
[functions.expression] use ConfigTree instead of ParameterTree
[functions.expression.base] remove unnecessary include, fix typo
[test.functions_*] adapt to the changes in the associated files