Hist balances should allow for only one transfer

[lajarre]

See [FIXME] in pd_inter_calc.py:

    if len(rows) < 2:  # [FIXME]
        return None

This results in not taking into account tokens which only have one transfer in the history of the given portfolio.

We should thus remove this if clause. As a consequence, the make_returns_df function should be made more robust.