make RDSE the default encoder for scalars

this creates a meta-class ScalarEncoder that provides (compatibility) constructor(s) for the "best scalar encoder"-implementation, should we want to change that in future again. 

Today's `ScalarEncoder` is renamed to `SimpleScalarEncoder`.

TODO:
[ ] what to do about all the tests, that fail due to the different inner-workings of RDSE/(Simple)ScalarEncoder ? 

Fixes: #1669 
Also, reading about `offset` param for RDSE, would it be useful to set it to (maxval-minval)/2 ?