Investigate a possibility of executing Gluecodium generators in parallel

[DanielKamkha]

In the current Gluecodium architecture all output language generators work independently of each other. They all use a LIME model as an input, but this model does not change during the execution. So there is a theoretical possibility of executing the generators in parallel.

Presumably, there is no shared state between the different generators. This needs to be investigated. There is a shared state outside of the generators, which is a "cache" mechanism. There are different options for dealing with that: getting rid of the cache entirely (#904), making the cache thread-safe, or doing better decoupling of the generator runs and cache writes.

It is also unknown whether the parallelization would bring any performance benefit. This needs to be tested.