ADD: added `comments` property to Network.
ADD: added `comments` property to Network.

todo: add read/write support for this property to and from
touchstone files

jhillairet/scikit-rf,willhaines/scikit-rf,willhaines/scikit-rf,willhaines/scikit-rf,hohe/scikit-rf,scikit-rf/scikit-rf,gibbons/scikit-rf,temmeand/scikit-rf,Ttl/scikit-rf,gibbons/scikit-rf,Ttl/scikit-rf,hohe/scikit-rf,aerialist/scikit-rf,gibbons/scikit-rf,scikit-rf/scikit-rf,temmeand/scikit-rf,hohe/scikit-rf,jhillairet/scikit-rf,temmeand/scikit-rf,jhillairet/scikit-rf,hohe/scikit-rf,aerialist/scikit-rf,Ttl/scikit-rf,willhaines/scikit-rf,Ttl/scikit-rf,aerialist/scikit-rf