#60 Help links to update

[Christopher Roland Ritson]

Can I suggest changing the help links as follows (or removing them):

• https://www.nilsliberg.se/ksp/easyabc/ becomes https://easyabc.sourceforge.net/
• https://www.stephenmerrony.co.uk/uploads/ABCquickRefv0_6.pdf becomes https://github.com/seisiuneer/abctools/blob/main/ABCquickRefv0_6.pdf

[Frederic Aupepin]

Updated in the last commit in github, will be included in the next release.