Prior to a recent commit (or this, if squashed), yield points of a non-linear
material with simple hardening were stored as three different properties. These
changes consolidate them into the new `YieldPoints` property.
String List instead of String should allow for arbitrarily many entries,
theoretically.
Note the defaults previously in `YieldPoint1` an `YieldPoint2` were arbitrary.
Now the list is kept empty by default.
Fixes issue #4720.
