Remove fractional unit assets

[tsmbland]

As of #1030, we allow some divisible assets to have a capacity not equal to the unit size. This occurs as a result of parent asset division when capacity of the parent is not a multiple of the unit size (e.g. capacity of 55 with a unit size of 10 -> 5 assets with capacity 10, and 1 "fractional" asset with capacity 5). This could represent, for example, half a wind turbine, which isn't very realistic.

This happens on two occasions:

• When pre-defined assets (i.e. from assets.csv) are commissioned as part of commission_new. Since users are allowed to put any capacity in the input file, which may not necessarily be a multiple of the unit size, and we don't want to make life overly difficult, I think we should just round up capacity to the nearest multiple of unit size, and raise a warning if this deviates by more than some tolerance
• When new assets from the investment algorithm are commissioned, since the investment algorithm works by optimising capacity of the parent asset, which, again, may not be a multiple of the unit size. As above, we could fix the problem of fractional assets by rounding up, but I think a more elegant solution would prevent this problem in the first place by restricting parent asset capacities to multiples of the unit size. I have a few ideas for this which I'll discuss in a separate issue (Rethink tranching for divisible assets #1044 )