Change slope bounds and number of loops in mud_calculator.py

[yucongalicechen]

Problem

In mud_calculator.py, the estimated slope reaches maximum when intensity is large

Proposed solution

• Increase the slope bounds
• Increasing the number of loops from 10 to 20 to achieve better results within a reasonable time frame
• Give a better docstring (change slit_width to half_slit_width to avoid confusion)