Truncated exponential sum

[mforets]

Write a function that computes the sum \sum_{i=i0}^{p} A^i * t^i / i!}. Note that if i0=0 one should use the exact computation of At + 1/2A^2 t^2 (matrix W).

• [x] outsource existing code to new function (#123)
• [ ] make code parametric in i0

[schillic]

... and use the function in the existing code base.