Milstein function

[psotom]

References to issues or other PRs

Describe the proposed changes

Added milstein function which computes the Milstein scheme for Stochastic Differential Equations (SDEs). Additionally, examples which show the functionality of the SDE solvers are added.

Additional information

Checklist before requesting a review

• [x ] I have performed a self-review of my code
• [] The code conforms to the style used in this package
• [x ] The code is fully documented and typed (type-checked with Mypy)
• [x ] I have added thorough tests for the new/changed functionality

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.90%. Comparing base (73161f7) to head (9ba66a5). Report is 30 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #615 +/- ## =========================================== + Coverage 86.65% 86.90% +0.24% =========================================== Files 156 158 +2 Lines 13322 13568 +246 =========================================== + Hits 11544 11791 +247 + Misses 1778 1777 -1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.