allow more complex growth functions to be applied to compound solver

[genedan]

See Vaaler & Daniel, problem 1.7.5. We can solve this by supplying a TieredTime to an Accumulation object, and then using the discount function.

I wonder if some people will try to use the compound solver in this scenario. The function does not support complex growth functions such as TieredTime, so it may be worthwhile to implement this feature. This is not high priority since there is already a way to do this using the Accumulation class.