carbonara: reduce the number of array copy
carbonara: reduce the number of array copy

By using `numpy.array.astype(dtype, copy=False)` we can limit the number of
data copy done in memory for faster processing and less memory usage.

At least in theory.

Change-Id: I348d5a7b818f8cbfe51e5a327f2dc7a5f8f355dd

gnocchixyz/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,sileht/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi