Add lifetimes again

[johannes-moegerle]

in #38 we removed all lifetime functionality for simplicity.
we should add this at some point again

[johannes-moegerle]

If you do so, check if the approximate lifetime calculations are still wrong (see also pairinteraction/ryd-numerov#1)

[johannes-moegerle]

added again in #31 and will be merged in main, once the v0.10a branch is merged
(we only added the "exact" lifetime calculations, since these are fast enough and we therefore dont need the approximate calculations)