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.
Merge pull request #504 from atgeirr/allow-no-spline-new-parser

Allow direct interpolation of props with new parser.