`Api_key` argument

[Nikoleta-v3]

Pass api_key as an argument

[NielsZondervan]

Not sure if we are referring to the same issue, but it would be nice if the user could set and get API keys from within the code. For example:

arcas.Ieee.set_key_api ('enter your API key here, will store it in key_api of arcas.Ieee')
arcas.Ieee.get_key_api ('no argument needed, will return API key from memory)

The same holds for the API keys from arca.springer and arca.nature
I would include the setting of API keys in Arcas example code and documentation since it will be the first thing users would need to do, also mention that arcas.Arxiv and arcas.Plos do not require an API key.

I would like to incorporate Arcas within our text mining methodology and template code (currently under review), preferably with the suggested settings of API keys. Otherwise users will blindly run the code and experience the APIError: status=403 without knowing why.