Calculate totals across nuclides for an extracted data frame

[gonuke]

All the data in an ALARADFrame should be from a table of data that already includes the totals for that table, and those total values should exist in the ALARADFrame.

Once a new data frame has been generated by filtering data from an ALARADFrame it may not contain an entry for a total across that data. However, there are limited cases in which it makes sense to perform such a total. Let's discuss here what those cases are and whether we want to provide a function or just allow users to perform that operation themselves.

[gonuke]

One case where it does makes sense, for example, is if a user filters all the isotopes of a given element and then wants to know the relative contribution of each isotopes. They will first need to know the total over all the included isotopes. If we provide a method to do this, however, there is a risk that a user invokes it with a dataframe that has multiple elements and the total may not make sense.