Abstract method to evaluate between two boundary values. Built-in
implementations all use linear parametric evaluation:
v = v0 + (v1 - v0) * fraction
Extenders of Evaluator will need to override this method
and do something similar for their own types. Note that this
mechanism may be used to create non-linear interpolators for
specific value types, although it may besimpler to just use
the linear/parametric interpolation
technique here and perform non-linear interpolation through
custom Interpolators rather than perform custom calculations in
this method; the point of this class is to allow calculations with
new/unknown types, not to provide another mechanism for non-linear
interpolation.