feat: muD fast calculation for values <0.5 and >6

[yucongalicechen]

Problem

Proposed solution

• Do not raise an error when muD < 0.5.
• Include muD up to 7 for fast calculation.
• Report user muD when we raise the error, e.g. "muD=8 is outside the range of 0.5<=muD<=7, please rerun the program..."

[sbillinge]

why are we going from 6 to 7? This seems pretty arbitrary. I think it is more a philosophical issue about how to handle this flow. We can define mud_threshold for example. Right now mud_threshold = 6. We could change mud_threshold to something else, but the conversation here might be better to be about what flow we want whenmud_threshold is exceeded in general.