Updated documentation in preparation to 3.1 Beta 0 release

git-svn-id: https://thermopower.svn.sourceforge.net/svnroot/thermopower/trunk@270 80daafeb-7430-0410-91d5-8b30b0bbd841
Added ObsoleteModel icons to old models no longer supported

git-svn-id: https://thermopower.svn.sourceforge.net/svnroot/thermopower/trunk@271 80daafeb-7430-0410-91d5-8b30b0bbd841