Share and vtk exporter both reset rate products

[emmacware]

I think rate products seem to reset to zero after a vtk exporting/storing, so subsequent storing methods register zeros for those products

[slayoo]

Thank you @emmacware

[slayoo]

The offending line is here:

PySDM/PySDM/products/impl/rate_product.py

Line 34 in de03b04

self.counter[:] = 0

This zeroing should be moved into notify()