Replace configuration concatenation operator + with *

[jagot]

We want to go towards unsorted configurations, i.e. where no ordering of the orbitals is enforced, and concatenation of two configurations then become an illdefined operation, c.f. c"1s 2s" + c"1s 2p" vs c"1s 2p" + c"1s 2s". If no canonical ordering is enforced, the results would be different between these two operations, which would make the + operator non-commutative. To reduce confusion, the idea is to replace concatenation with + by *. This would also be more in line with Julia's string concatenation.