Support reading of lists in custom xyz-files

[YHordijk]

Currently key=value pairs only allow single values. We can extend the syntax to support lists as values as well. I propose we add support for the following:

12

C       0.00000000       1.14206764       0.90887116
C       0.00000000       1.19134753      -0.47779047
C       0.00000000       0.00000000      -1.18863009
C       0.00000000      -1.19134753      -0.47779047
C       0.00000000      -1.14206764       0.90887116
N       0.00000000       0.00000000       1.58935329
H       0.00000000       2.05493274       1.49740370
H       0.00000000       2.14479192      -0.99199860
H       0.00000000       0.00000000      -2.27369478
H       0.00000000      -2.14479192      -0.99199860
H       0.00000000      -2.05493274       1.49740370
Xx       0.12739120      -0.00077609       2.50049043

conn = 6, 12
active_atoms = 8

Currently conn will be read as '6, 12'. I think splitting on , will be the easiest way to create lists. We can then simply split the value by , first, and then parse each part again in a small list-comprehension. This should then yield conn: list[int] = [6, 12]