Fix invalid escape sequence warnings in KCI and Kernel docstrings

[bessx]

Fix invalid escape sequence warnings in KCI and Kernel docstrings

This PR fixes SyntaxWarning: invalid escape sequence '\e' that appears on Python 3.12+ / 3.13 when importing causallearn.

Problem

Some docstrings in KCI.py and Kernel.py include LaTeX-like math expressions such as \epsilon.
In normal Python strings, \e is treated as an escape sequence, which is invalid and now triggers a warning:

.venv/lib/python3.13/site-packages/causallearn/utils/KCI/KCI.py:500: SyntaxWarning: invalid escape sequence '\e'
  With this we could save one repeated calculation of pinv(Kzy+\epsilonI), which consumes most time.
.venv/lib/python3.13/site-packages/causallearn/utils/KCI/Kernel.py:45: SyntaxWarning: invalid escape sequence '\e'
  Centers the kernel matrix via a centering matrix R=I-Kz(Kz+\epsilonI)^{-1} and returns RKR

Solution

• Converted affected docstrings into raw strings (r"""...""").

Impact

• No code or functional changes.
• Silences warnings on Python 3.12+ and improves readability of the math in docstrings.
• Keeps the intended LaTeX-style formatting intact for users and documentation tools.

[kunwuz]

Thanks, that looks good!