Implement slippage for sub-daily resolutions

[TristanFecteau]

We currently only support slippage for strategies with a DAILY resolution.
This is because slippage needs to get the daily high price to calculate the total slippage. For daily resolutions, we use fmp's historical-price-full endpoint wich takes into consideration split and dividend where as we take the historical-chart endpoint for sub-daily resolutions and this endpoint does not take into consideration split and dividend. This makes the daily high price not correlate with the hourly/minute prices. If you want an example of this :
https://financialmodelingprep.com/api/v4/historical-chart/AAPL/1hour?from=2019-03-05&to=2019-03-05&apikey=YOUR_API_KEY
https://financialmodelingprep.com/api/v4/historical-price-full/AAPL?from=2019-03-05&to=2019-03-05&apikey=YOUR_API_KEY

Things to do :
-Buy a paying subscription ( starter would do)
-Change the sub daily endpoint for historical-price-adjusted
-Raise an error if the api_key is free and the strategy is sub daily (fmp will return an error : Special Endpoint : this endpoint is only for premium members)
-Remove the comments in portfolio._get_daily_high_price and remove the if that only caluclates slippage if Resolution is DAILY in _get_daily_high_price

[TristanFecteau]

@SamiMammouche would be nice to find a get address where we put our api_key and receive the subscription plan for that key (Kinda like we do for the remaining fmp calls). Would make this issue 10 times easier.

[TristanFecteau]

Only export number of calls if free subscription to FMP