Add checking to contour_threshold for creating various masks

[amoodie]

A point that has tripped me up a few times now is that the value to contour_threshold needs to be adjusted when creating a mask via the MPM methods. I.e., the default value of 75 is outside the range of the values in the MPM mean_image, which is always in [0, 1].

Two ideas, both of which would be helpful, I think.

1. Change the setup so that the default value when creating via MPM is 0.75 and when creating via OAP is 75. Add this to documentation and testing and it would be fine. Regardless of the default value, another value can always be supplied.
2. When going to extract the contour in the mask creating bits here, add a check that one and only one contour has been found. If 0 or >1 are found, raise an error and stop. Possibly suggest increasing the max_disk value in the error?. Related: if 0 contours are found, return the range of the values in the error, which might be helpful.

issue created by discussion in #104