vlsvReader get_variable_length consolidation

[alhom]

Several functions use a test-variable read or such to check the length of a variable (1 for scalar, n for n-vector..). This is not entirely trivial when accounting for numpy maskedarrays:
#247

There exists a function that checks the vector length from file reads:

analysator/pyVlsv/vlsvreader.py

Line 735 in 038dadc

def read_variable_vectorsize(self, name):

Consolidating such checks to a single function, with datareducer support, and applying that function wherever required would do good.

[alhom]

Pythonic exception handling such as try len() except len=1 might be good solution.