Decide what to do with missing and unknown values

Always store data in JSON "as is" should remain a basic DB.nomics principle. Consequently fetchers should never complete missing values in series. This matter should be resolved at the API level. (decision adopted during Committee Meeting 2017-09-06)

We should avoid problems existing in the previous UI: missing values are sometimes not interpreted as such.

Propositions to solve problems related to missing and unknown values:

• #62 (closed) Give a unique symbol to missing values – S
• #95 (closed) Store in the metadata of a series the values that should be interpreted as 'missing' (e.g. NaN, N/A, Null, -1, 9999, etc.)
• Check the existing plugins to make sure that they always return "full" time series with missing values (except for daily data).
• A common missing value code (NA?) should be decided for the UI.
• For the API, the missing code should be consistent with the usage in the target software (R, Matlab, etc...)