FFTW Poisson solver and tile geometry

[fjansson]

The FFTW Poisson solver doesn't handle all tile geometries well. Problematic cases are

• odd itot and/or jtot,
• itot not divisible by nprocy
• jtot not divisible by nprocx

Fixes proposed in 8f2c585725b0903334b . @jiskattema and @v1kko could you have a look at them? There are some more details in the commit message. (@CasparJungbacker FYI)

Test case: BOMEX or RICO with the following grid settings

&RUN
...
nprocx     = 3
nprocy     = 2
/
&DOMAIN
itot       =  21  ! 3*7 
jtot       =  22  ! 2*11
kmax       =  97  ! prime
xsize      =  2100
ysize      =  2200

[fjansson]

The fixes have been merged, in main and in dev.