[PLAT-6666] Linear interpolation instead of simple average

Calculation of the rate of a stub period by interpolating between two indices: interpolation should be linear and not simple average.

@mclornp @stuartw  Would you take a look?

Here we don't care about the denominator. I just needed to count the actual number of days between these days to calculate the weight properly. That's the way I came up with - if there is another way, I'm interested.