backend: QTYPE_FIXEDPOINT, add missing logic for question.decimal_places

[RoboSparrow]

As of now, the serialisation/deserialisation of QTYPE_FIXEDPOINT is exact the same as for QTYPE_INT
Paul provisioned the field decimal_places in struct question but it is not utilised yet. The decimal info is also not passed on to the client nor stored in the answer

The idea of this type is that the question requires (and validates) a defined accuracy, otherwise it would be just a free float we need to parse at the backend (count decimal point)

This is a special scenario for the serialisation deserialisation

{
  value: 314  // lonlon
  decimal_places:0 // int
} 
json number: 314 // division by zero!
serialised answer: "314"

{
  value: 314  // lonlon
  decimal_places: 100 // int
} 
json number:  3.14
serialised answer: "314"

{
  value: 314  // lonlon
  decimal_places: 1000 // int
} 
json number:  0.314
serialised answer: "0.314"

affected:

• new serialise()
• new deserialise()
• answer_set_value_raw()
• answer_get_value_raw()
• python next_question()?