bugfix: use package amssymb for \square
replace \Diamond by \lozenge

as recommended by in sec. 3.2, p.4, Downes M., Short Math Guide for LaTeX, AMS, v1.09 (2002-03-22).
The same recommends using \square instead of \Box.
In any case it verifies what is visually observed: that \lozenge=\Diamond and \square=\Box.
use ltlfonts to replace custom definitions that use amsmath

because \LTLdiamond is a rotated square, \LTLcircle have better vertical centering, and all operators are bolder (holds also if we rotated \Box), among other differences.

The only drawback for now is that ltlfonts are not yet included in TeXLive's package repository, so have to be manually installed instead.
update copyright notice
bugfix: include missing package dependencies