Add logic to avoid reallocating ZCOMBUF[RS] at each call

[samhatfield]

This is a significant optimisation of the CPU code path. Credit owed to @marsdeno.

TCO1279, 48-node benchmark (--norms --truncation 1279 --niter 100 --nlev 137 --nfld 1 --vordiv --uvders --scders -v):

• develop:

  Inverse-direct transforms
  -------------------------
  avg  (s):   0.4258
  min  (s):   0.3726
  max  (s):   1.2771
  med  (s):   0.4168
  loop (s):  50.9419

• pre_allocated_buffers:

  Inverse-direct transforms
  -------------------------
  avg  (s):   0.2227
  min  (s):   0.1793
  max  (s):   1.1176
  med  (s):   0.2128
  loop (s):  30.9310

Almost 2x speed-up of the median transform time with identical norms.

[marsdeno]

Looks good to me