Allow for creating new assets from processes and adding to pool

[alexdewar]

It may be that we can just use the existing Asset::new constructor for creating new assets, in which case we can just add a method to AssetPool for adding new assets.

The bigger problem is that the data structures in AssetPool were written assuming that all the assets were defined ahead of time in assets.csv, so it won't be just a case of appending an Asset to a Vec.

(I don't have time to finish this issue description now, but prod me if you need more info!)

[alexdewar]

We should probably do #504 at the same time, while we're reorganising Asset anyway.

[alexdewar]

From the looks of things, we should have all the necessary info to create a new Asset from a given process. We will need to calculate capacity as per the description in the docs.