Error while loading shared libraries: libicui18n.so.60 (magfieldmapper_gui)
The mentioned libicui18n.so.60 library was contained in package for 18.04 LTS.
You can download and install it manually on 20.04 LTS using commands below:
cd ~/Downloads
wget http://security.ubuntu.com/ubuntu/pool/main/i/icu/libicu60_60.2-3ubuntu3.2_amd64.deb
sudo apt-get install ./libicu60_60.2-3ubuntu3.2_amd64.deb
and then retry to run ./magfieldmapper_gui.