add lumped element definitions to materials

This takes the lumped elements and adds them to their designated material definitions. Standard materials and lumped elements go into two separate mixing tables in PyARC: mixing_table and lumped_mixing_table, respectively. In order to combine the lumped elements into the openmc material definition, three steps are needed:

1. make a mapping of which PyARC materials have lumped elements in them
2. update the total atom density of a material that has a lumped element
3. add components from each mixing table to the openmc material.

I have one clarifying question though: I think the lumped elements can only be used in materials that are defined by adens correct? If not, then the logic may be a tad more complicated.