Make Candle's compute_init set a boolean reference

[myreen]

At the moment, Candle's compute_init checks the characteristic equations but the fact that they were checked is not remembered. As a result, each call to compute checks the characteristic equations again, which would be unnecessary if Candle remembered (using a boolean reference) that the characteristic equations have been checked before.

This issue is about adding a boolean reference and offloading the checking of characteristic equations from compute.