Fix logic to choose ObservationConvention

[jonatan-ivanov]

There are two issues with the logic that chooses and applies the ObservationConvention on the Observations.

1. The logic can choose and apply multiple conventions, also in one case it can mix them together
2. In one case the logic pics the wrong convention

Here's the old and the new decision matrix:

OLD:

1. custom     default     no pre-cofigured  -> custom
2. custom     default     pre-cofigured     -> custom on top of pre-cofigured
3. no custom  default     no pre-cofigured  -> default
4. no custom  default     pre-cofigured     -> default on top of pre-cofigured (*)
5. custom     no default  no pre-cofigured  -> custom
6. custom     no default  pre-cofigured     -> custom on top of pre-cofigured
7. no custom  no default  no pre-cofigured  -> local
8. no custom  no default  pre-cofigured     -> pre-cofigured

(*): This logic is wrong: 1. they are mixed, 2. pre-cofigured should have choosen

NEW:

1. custom     default     no pre-configured  -> custom
2. custom     default     pre-configured     -> custom (not really a valid case, use custom)
3. no custom  default     no pre-configured  -> default
4. no custom  default     pre-configured     -> pre-configured
5. custom     no default  no pre-configured  -> custom (providing default is recommended)
6. custom     no default  pre-configured     -> custom (providing default is recommended)
7. no custom  no default  no pre-configured  -> local names/tags will be used
8. no custom  no default  pre-configured     -> pre-configured

cc: @bclozel