Time refactor tidying

[cliffckerr]

Issues remaining after https://github.com/starsimhub/starsim/pull/623

• [x] #636
• [ ] #642
• [x] #648
• [ ] Interoperability with pd.datetime etc: https://github.com/starsimhub/starsim/pull/623#discussion_r1771329650
• [ ] Match objects, not strings: https://github.com/starsimhub/starsim/pull/623#discussion_r1771340770
• [ ] Perform appropriate type checks and conversions: https://github.com/starsimhub/starsim/pull/623#discussion_r1771347779
• [x] "It would be nice if the result of these operations was also a TimePar - but not sure if that's feasible from a performance standpoint"
• [x] Reconsider use of parent
• [ ] Rate times duration should give number
• [x] "Instead of timevec, timearray, and dt, I'm thinking we should have tvec (in module units) and abs_tvec (in sim units, starting at 0), and dt (in module units) and abs_dt (in sim units)."
• [ ] Rename timevec to tvec
• [x] Give timepars a to() method, e.g. ss.dur(30, 'days').to('year')

[cliffckerr]

Haven't finished all these tasks, but the rest can be done after v2.0.