Use squeletonization to close myelin

[jcohenadad]

As pointed out in #397, there is a need to manually correct myelin masks, including closing them so that the flood fill algo works. There might be ways to "close" myelin masks automatically after a squeletonization algo. To investigate...

[Stoyan-I-A]

For some visualization:

[mathieuboudreau]

@hermancollin this old issue may be relevant to the user issue you were discussing today.