Fix/improve Factor Observation JSON serialization

[Affie]

Ideally the packed observation should be [de]serialized directly to JSON and not include the extra string step with manual type creation.

• First wait for [BREAKING] Package rewrite for a 1.0 release JuliaIO/JSON.jl#374 and maybe upgrade all serialization to JSON.jl in one go.

[dehann]

observation should be [de]serialized directly to JSON and not include the extra string step with manual type creation.

Agree

First wait for JSON.jl 1.0

Agree, JSON.jl would be nice. Hopefully that drops soon...