Adding OECD endpoints

[jmaslek]

Pull Request Template for OpenBB Developers

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1. Why? (1-3 sentences or a bullet point list):

   • Sinan asked for endpoints:

• CPI

2. What? (1-3 sentences or a bullet point list):

   • Added CPI from oecd which doesn't require an api key - but the input parameters are a bit more confusing. Nothing like 29 possible expenditures.
   • Edited the standard model
   • Expanded cpi -> consumer_price_index in files

3. Impact (1-2 sentences or a bullet point list):

   • I did change the standard model to be compatible between fred + oecd, but the default values should work.

4. Testing Done:

Working on it