Trial inspect call for emitting warnings

[alhom]

Re: #276

This emits a warning when read_variable is called from CLI or from __main__ i.e. a user script not included in the analysator modules. This probably includes everything under scripts/ though, not sure if I like that. I think those are more user-facing, so it does serve a purpose there.

Testable via e.g.:

import pytools as pt
fn = 'somebulk.vlsv'
f = pt.vlsvfile.VlsvReader()
f.read_variable('vg_rank')

f.read_interpolated_variable('vg_rank', [0,0,0])