Allow direct interpolation of props with new parser.

After transitioning to use the new parser, the SinglePvtDead class was never used even when the 'samples' argument used to control usage was zero or negative. The resulting construction of SinglePvtDeadSpline objects was then failing.

This change adds a new constructor to SinglePvtDead, and restores the ability to control spline usage with the samples argument.

Note that the three constructors for the class are very similar, resulting from copying and pasting. This PR does not refactor this.