Improve PolyConf gen_pairlist() error handling #96
Description
Gen_pairlist doesn't check whether the atoms exist, which means you can generate spurious pairlists that don't match your polymer.
Eg, for an alkane polymer of length 25, I can run gen_pairlist for some pair of atoms (eg J=CA,K=C) from first_resid=1 to final_resid=50, and the resulting pairlist will exist. This will only fail when the user tries to use the pairlist with solver, and it hits a resid that does not exist.
I need to consider how best to handle this. Pairlist operating without needing the polymer to exist is very convenient, but some validation might be useful.