dataset.py file structure enforcement and MLEM mask

[jadball]

Apologies for many lines of changes, my code editor auto-linted things.
The changes to dataset.py are outlined:

1. Added update_paths() method to DataSet class
   This function determines self.analysispath and a set of paths to directories and files such as self.pksfile and self.grainsfile
   This function is called during self.__init__() but also at the end of self.load() because the paths depend on self.sample and self.dataset which are not defined when the main load() function is called (because an empty DataSet() object is created and afterwards loaded from file).
2. Added a default h5 file path to the self.save() and self.load() methods.
   If no h5 path is manually provided, h5name is set to self.dsfile_default which is calculated in self.update_paths()
   At the end of self.save() and self.load(), we know the file saved successfully, so we update self.dsfile equal to h5name (so we can keep track of where we saved to)

Only one actual change to roi_iradon.py :

1. Added mask capability to MLEM function
   This mask is not given to the initial sensitivity_image iradon call because we invert the result which means dividing by zero.
   The mask is instead just applied right at the end before returning mlem_rec
   This avoids weird corner streak artifacts observed when running MLEM by 15+ iterations.

[jonwright]

Looks good to me, Thanks!
For the formatting - it would be great to figure out how to add "black" as a commit hook so that this gets sorted out magically.