Speeding up admiral code

[manciniedoardo]

Some of our functions, e.g. the *_joined ones, can be a bit slow, especially with large input data. It may be good to explore ways of speeding them up. Possible options include:

• Using data.table
• Implementing chunking/parallelisation approaches

[bundfussr]

We should also analyse the code on unnecessary steps. When I worked on the memory issue in the *_joined_*() functions I noticed that we spend considerable time on checking on duplicates and that the checks are performed although we know that there are no duplicates.