Adding functions for cir simulation and estimation

[Blunde1]

Appendix B: Split-profiling approaches a CIR process asymptotically

Adding functionality that accounts for this**

cir.hpp

• rmax_cir: simulate vector of maximum cir observations at $\tau = f(u)$
• expected_max_cir: numerically calculate expected maximum of $m$ maximums
• expected_max_cir_approx: fast estimate expected maximum of $m$ maximums
• estimate_shape_scale: fit observations of maximums to a gamma distribution approximately
• interpolate_cir: interpolate cir on $u$ to cir on pre-defined grid for fast calculations

node, gbtree and ensemble classes have pending updates

[Blunde1]

All checks passed. Leave open over the night to think about possible errors.