Better description or conceptualization of gsa_dict

[cmutel]

This could be much easier to use, especially for people who aren't well-versed in the literature.

1. Think hard about whether you want to continue to use X, etc. variable names. I don't think we lose anything by being a bit more descriptive.

2. I go back and forth on this, but my conclusion right now is that this object should be a subclass of dict:

from collections import UserDict

class GSA_Dict(UserDict):
    def __str__(self):
        return "GSA_Dict instance with the following keys:\n{}".format("|".join(key for key in self))

This subclass a) makes it clear that this is a "special" object, b) allows for nicer string representation in the shell, and c) allows you to add custom methods, if desirable, in the future.

3. There is a lot of code like:

self.gsa_dict.update({'X': self.generate_samples()})

Is there a reason that we can't just say:

self.gsa_dict['X'] = self.generate_samples()

This is a lot easier to parse, at least for me.