Changing `insolation` attribute of `climlab.radiation.RRTMG` object doesn't work

This should work but doesn't:
```
import climlab
state = climlab.column_state()
rad = climlab.radiation.RRTMG(state=state)
assert rad.insolation == rad.subprocess['SW'].insolation
rad.insolation *= 1.01
assert rad.insolation == rad.subprocess['SW'].insolation
```
The new value is not propagated to the shortwave subprocess.