Moved mpl style to optional file not imported by default

[matpompili]

As of now, ARC forces a non-default matplotlib style when imported.
I moved the custom style to a non-automatically-imported file (arc.mpl_style), such that it can still be used if needed, but it does not override the style in projects using ARC.

[nikolasibalic]

This would break how things are showed now in many places. I would rather suggest that if default style is needed, the following two lines are added after ARC import to force default style.

from arc import *
import matplotlib as mpl
mpl.rcParams.update(mpl.rcParamsDefault)