telescoping initial guesses for CC

[jeffhammond]

All the iterative CC methods appear to use MP2 as an initial guess. I'd really like to be able to telescope out e.g. MP2 ➡️ CCSD ➡️ CCSDT ➡️ CCSDTQ.

Any chance this is implemented already? If not, this issue serves as a feature request. I'll try to implement it over the holiday if I can find time.