Fix set_freq

Fix the set_freq function and add some functionality:

• forward fill when upsampling
• use method parameter of the asfreq method in set_freq
• test cases for set_freq module

Fix Requirements Again

• remove reading of requirements from files and use direct list instead

Fix Requirements

• limit-order-book is not required

Log Returns

• add log_return parameter to the cash_returns method

Initial Release

Calculations of:

• cash return
• percent return
• volatility