energy_mixes: Function to randomly generate a mix of energy from different...
In saraorofino/test:

Function to randomly generate a mix of energy from different sources to meet the global energy demand

1	energy_mixes(energy, demand, probs)

`energy`	a vector of the names of different energy sources (i.e. s = solar, w = wind etc)
`demand`	the amount of energy (in TWh) that you want to draw (i.e. the global demand)
`probs`	the probability of drawing each type of energy based on global energy potential for each source

out a data frame consisting of the following items

energy_type: The different energy sources
TWh: The number of TWh provided by each energy source
percent_of_demand: The percent of total demand that comes from each energy source