Models shouldn't be processed multiple times

[christuart]

At present, the software reads in the .sections, .nodes and .elements files every single time the same model is used, which involves the O(n^2) (at least?) task of determining element links. Instead, a new format should be saved which includes this data, and that should be loaded if available. It might be wise to save in this file an md5 of the contents of .sections, .nodes and .elements in this new format, and use that to check whether to rebuild each time.