Refine ALLOCATE AVAILABLE search to converge under broader conditions

[ToddFincannon]

The ALLOCATE AVAILABLE C implementation searches the x axis for a value that allocates available resources according to the given priority profile (see #100 for details). The current algorithm worked well for the example model in the Vensim docs, but fails to converge for two much larger priority profiles in EPS.

[ToddFincannon]

The problem turned out to be that the spread of priority values (means) in EPS is much larger than the example. The search algorithm needed to skip over much bigger steps in the beginning to find the x value where the allocations converge. I scaled the initial search delta to the spread of the means to get the right size. I also needed to increase the allowed number of search steps to 100, since the algorithm needed as many as 60 steps to converge in EPS. This is still a pretty fast process.

I added an optimization that skips the normal curve calculation if the requested quantity for a requester is zero.