def register_return_func(func):
SETTINGS['<STR_LIT>'] = func<EOL>
Registers the 'returns_func' that will be called for
retrieving returns data.

Parameters
----------
func : function
    A function that returns a pandas Series of asset returns.
    The signature of the function must be as follows

    >>> func(symbol)

    Where symbol is an asset identifier

Returns
-------
None