Suggestion: simplify constructor for sliceable dicts

[matthew-brett]

I rather like this simplification of the sliceable dict classes, because it
strips them down to only what they are meant to do - allow slicing per element
as well as key access. This PR is to ask you what you think:

Try dropping the special cases for the contructors for sliceable dicts,
and deal with None as input value in the tractogram properties.

[MarcCote]

I like the simplification too. I don't remember why I was specificly checking if the args[0] was a sliceable dict in the constructor. The tests don't seem to complain. Do you know if self.update(dict(args[0])) does something fancy compared to self.update(args[0])? I do remember that LazyDict needed special care though because of accessing the element of the lazy dict returns a generator instead of the function that creates that generator.

[matthew-brett]

Looking at the update method of MutableMapping, the first and only positional argument should be something that generates a dictionary, either a dictionary / mutable mapping, or a sequence of tuples. Then all keyword arguments are inserted as keys into the dictionary.

So, the previous code had this for a SliceableDataDict:

self.update(**args[0])

With a single SliceableDataDict as positional arg to `init``, this PR leads to:

self.update(dict(args[0])

I think, because the SliceableDataDict is a mutable mapping, these will lead to the same result - in the original code the key, value pairs get passed as kwargs, in the new code they get passed as the positional argument.

[MarcCote]

Great. Is it ready to be merged?

[matthew-brett]

Yes, ready for merge.