Check the existing historical GDP parameter first

I find the line in `read_data() `below checks whether `gdp_calibrate ` (imported from external excel input files) has at least 2 periods before the model horizon.

https://github.com/iiasa/message_ix/blob/2045b07c13322ce0224ce809ca1c64c1e5f64398/message_ix/macro.py#L275-L286

But this can be also imported from `self.s.par("historical_activity", {"technology":"GDP"})`. So the condition can first check the parameter and, if so, import historical values directly. If not, then it can ask for external historical GDP inputs.

Start in message_ix/macro.py at read_data(), especially lines 275-286, and inspect how historical_activity for technology GDP and external gdp_calibrate inputs are represented. Confirm the existing parameter is checked first, with external historical GDP input requested only when it is unavailable or insufficient; verify the surrounding model behavior after the change.

Written by the indexing model from the issue text.