Allow users to omit demand slicing information for uniform demand across the year

[alexdewar]

Currently if a user provides demand slicing information for a process for at least one time slice, they must provide information for all time slices (possibly with entries covering whole seasons etc. -- the important thing is all time slices are covered).

However, the user may simply want uniform demand across the year and it seems a little perverse to make them supply all this information in demand_slicing.csv. We could just have a rule that if the user doesn't provide demand slicing for a process for any time slices, then we assume demand is uniform (i.e. just proportional to time slice length).

This does raise the possibility of a situation where no demand slicing needs to be specified at all. At present we don't allow users to provide empty CSV files (#106), so in this case we would have to require that they not provide the file at all.