Allowing blended fuel sources

[shimwell]

Currently just 'DD' or 'DT' sources are allowed using the fuel: str = "DT", argument

We could add sources that have a fraction of DT and a different fraction of DD with a more flexible input for fuel.

How about fuel also accepts a list of tuples, where the first entry is 'DD' or 'DT' and the second entry is the relative strength

fuel = [('DD', 0.1, ('DT',0.90)]

or is a dictionary better

fuel = {'DD':0.1, 'DT':0.9}