[feature request] allow connectivity data to be (more) lazily loaded

[xgui3783]

as of 8d11569

code to reproduce:

import siibra
fs = siibra.features.get(
  siibra.parcellations['2.9'],
  "streamline lengths"
)
f = fs[0]
f.get_matrix(f.subjects[0])

expected result:

only downloads the relevant csv file

actual result:

downloads the entire zip archive (~300MB)

related to #190

[AhmetNSimsek]

This is on my to-do list. It is a general issue how we deal with zip files. (the issue lies in zipfile property.)
Relevant branch: https://github.com/FZJ-INM1-BDA/siibra-python/tree/refactorZipfileConnector