## Delayed neutron precursor decay rate tally

*Created by: samuelshaner*

This PR adds a tally for the delayed neutron precursor decay rate (also called the decay constant). This tally is important for kinetics analysis with any cross section library that does not have the same decay rates array for each nuclide (e.g. nndc). Some important notes about the implementation of this tally:

- The tally only supports an analog estimator since the decay rate for a specific nuclide is needed in the tally. I guess one could loop over all the nuclides in a material for a track length tally, but that seems onerous.
- The decay rates in the nndc ace files seem to be in units of
*shakes*and not*seconds*. This was deduced by looking at the tallied decay rates and not any documentation. I searched around for any reference of the units on the decay rate, but could not find a definitive answer anywhere. If anyone has any info on whether there is a difference in units in different libraries, please do share! - I'm not positive on the correct procedure to tally the decay rates. I chose to tally the delayed-nu-fission weighted decay rate as delayed fissions produce delayed neutron precursors. This would effectively weight the decay rate by the delayed neutron precursor production rate. However, another option would be to tally the mean lifetime of a delayed neutron precursor times the delayed-nu-fission rate and then invert this to get the decay rate.
- I implemented a MDGXS for the decay rate and added an example of computing decay rates to
`mdgxs-part-i.ipynb`

. - The documentation and test suite have been updated.
- This PR builds on PR #711