Fix ordered pointcloud2 format

[doronhi]

related to #1664
based on @joelbudu's #1668, set _msg_pointcloud.width before resize.
I prefer this implementation as it's shorter and contains fewer if-else cases.
If you approve, I'll try to add a unit-test based on the example from @mferenduros before merging.

[joelbudu]

@doronhi, I agree this is a much more elegant solution. The unit-test would also be a great addition. Thank you

[doronhi]

Couldn't find a wrapper for pcl::fromROSMsg in python and since my current unit tests are in python a unit-test for this will have to wait a little longer. Merging as it is.

[joelbudu]

Thanks for this @doronhi . Is it possible to merge into the eloquent branch as well? I know it's EOL but it's what we're using currently for Ubuntu 18.04. Would be grateful.