lbmpy: various small improvements

- getShearRelaxationRate is a free function now -> works also with cumulant methods
- better error message when calling kernels with wrong or too few parameters
- entropic & incompressible is not working by default due to pdf shift -> added NotImplemented exception
- new creation function for 'raw_mrt' where all relaxation rates can be independently specified
- enhanced entropic creation funtion, supports omega output field now