distributions tracking discretizations

[sbenthall]

From #720 , @llorracc :

I have proposed a modest modification to the way Dolo does things, which is that the discretized object should keep track, inside itself, of the way it was made. Like, it should know that it is a 7 point equiprobable discretization of a censored normal distribution with censoring points at 2 standard deviations above and below the mean. That way, we can pass the discretizations created in one place to be used somewhere else, without losing track of how the object was created.

[sbenthall]

superseded by #730