periodic_in_z not working about pysph

[sankasuraj]

test_periodic_3dnnps.py.zip
Attaching a better test case

[meyer-nils]

Lines 490 and 491 in nnps_base.pyx are as follows:
# reset the length of the arrays
x_low.reset(); x_high.reset(); y_high.reset(); y_low.reset()

I am pretty sure, one should add a reset for the z-component as well.
Changing it to
# reset the length of the arrays
x_low.reset(); x_high.reset(); y_low.reset(); y_high.reset()
z_low.reset(); z_high.reset()
resolved my issues with z-periodicity and successfully passes your tests. Note that you need to recompile cython files by running
python setup.py develop
again.

[prabhuramachandran]

@sankasuraj -- thanks.
@nilsmeyerkit -- thanks, I'll fix this in master tonight.