read_velocity_cells optimization

[alhom]

read_velocity_cells (

analysator/analysator/pyVlsv/vlsvreader.py

Line 3392 in 5091495

def read_velocity_cells(self, cellid, pop="proton"):

) reads in the sparse VDF data and populates and returns a dict of v-space index -> v-space value. However, all uses of read_velocity_cells look to be instantly converting the dict keys and values to lists for further processing.

Therefore, there's no need to build a dict here, and that part can be optimized away.