consistency with json functions

[ascheidl]

to serialize a grid to a json string the function to_json is used, however to deserialize the function from_json_string is used ... we should either introduce to_json_string or remove from_json_string and include this functionality in from_json

[KS-HTK]

Additionaly if this is not changed, atleast the function from_json_string should be documented. The docs have no mention of this function.