Cell width used in PMFT does match max_x, etc.

[Eric Harper]

In order to be able to actually sample the entire area given by [-max, max], the cell width must be norm(dot(max, max)). This value is not available to the user, so cell width errors are not immediately obvious. the used value should be made available to the user.

[Eric Harper]

Might be worth it to check at the python level and error out before hitting the more opaque C++ level issue.