Aggregate credit ratings information into fewer rating categories

aggregates the credit ratings information stored in the
`totalsGrouped`

= transprobgrouptotals(`totals`

,`groupingEdges`

)`totals`

input into fewer ratings categories, which are
defined by the `groupingEdges`

argument.

[1] Hanson, S., T. Schuermann. "Confidence Intervals for Probabilities of
Default." *Journal of Banking & Finance.* Vol. 30(8),
Elsevier, August 2006, pp. 2281–2301.

[2] Löffler, G., P. N. Posch. *Credit Risk Modeling Using Excel
and VBA.* West Sussex, England: Wiley Finance, 2007.

[3] Schuermann, T. "Credit Migration Matrices." in E. Melnick, B. Everitt
(eds.), *Encyclopedia of Quantitative Risk Analysis and
Assessment.* Wiley, 2008.