activating BLAZE part way through a run sequence

[har917]

The BLAZE fire module (developed with a requirement for POP to be active, so requires multiple spin up sections) operates in two main areas of the code - climate (to calculate fire weather metrics and fire climatologies) and carbon cycle (CNP, POP).

Conceptually, with the current implementation, there is an unwanted potential that either

• if BLAZE is activated during the spin up of any run, we lose all vegetation/prevent creation of vegetation
• if BLAZE is not active during the spin up, BLAZE cannot be subsequently reactivated since the climate metrics aren't available in the associated restart file (climate_rst.nc).

We need to follow this through and determine whether we need to add capability/change existing capability to break this catch-22.

@har917 to discuss with @nierad

[nierad]

In LPJ-GUESS BLAZE is turned off during spinup for the first 200 years. And with "turned off" I mean it is skipped for that exact reason.
I have no idea how long the spinup for CABLE/CASA normally takes but having a few generations of trees (100 yrs) grown without fires and then switch it on seems a reasonable thing to do.