Create data ID aliases

For many applications using large sets of data shared by several users, to use an automatically generated ID to retrieve a data is difficult or even impossible. DAGDA allows the user to define data aliases, using human readable and expressive strings to retrieve a data ID. Two functions are defined to do it:

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dagda_data_alias(const char* id, const char* alias):
Tries to associate ``alias'' to ``id''. If the alias is already defined, returns a non zero value. A data can have several aliases but an alias is always associated to only one data.

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dagda_id_from_alias(const char* alias, char** id):
This function tries to retrieve the data id associated to the alias.