Minor/reduce memory consumption associated with c_rate

• Add some comments about c_rate type

• Guard against using c_rate based on heating flag

• Don't allocate c_rate if heating is false

This large array is only populated if heating = .true. and is only used if write_heating is true. The array is ntheta*nx*ny*nspec*3 large and is not distributed. Hence it is the size of a distribution function shaped array multiplied by 3 * nproc / (2*nlambda*negrid). For runs with nlambda~64 and negrid~16 it therefore exceeds the size of a distribution function with nproc > 680.

It’s possible that we could further halve the memory cost of c_rate by declaring this as real. This is currently blocked by the use of c_rate as an out argument in some calls to integrate_moment.