Provide a default value for obs_sim in chain_ll()

[jamesmbaazam]

I think a sensible default for nsim_obs in chain_ll() would be useful. Without detailed domain knowledge for these things it can be hard to judge a sensible magnitude and changing will scale the run time. Should it be 100, 1000, 1,000,000? By giving a sensible default it will help beginners and doesn't prevent experienced users from modifying. It is also backwards compatible.

Originally posted by @joshwlambert in #79 (comment)

[jamesmbaazam]

It is not possible to determine a sensible default of the said argument as it depends on the context. This issue will therefore be closed.