Issues with resolution other than mMR

[ashgillman]

G'day Casper,

Loving this tool, thanks.

There are a few issues with using resolutions other than mMR.

The Shape class should cast its calculations to type np.int (and floor?)
Otherwise I run into type issues here: https://github.com/casperdcl/brainweb/blob/master/brainweb/utils.py#L301

Somewhat related but not really: the noise() function assumes sigma is given in voxel units - is this preferred? Potentially confusing for the user, we had to change sigma with resolution.

[ashgillman]

Happy to make a PR if needed