XINDEX - determine index of x with respect to an array of x values

xindex		determine index of x with respect to an array of x values

Input:
nx		number of x values in array ax
ax		array[nx] of monotonically increasing or decreasing x values
x		the value for which index is to be determined
index		index determined previously (used to begin search)

Output:
index		for monotonically increasing ax values, the largest index
		for which ax[index]<=x, except index=0 if ax[0]>x;
		for monotonically decreasing ax values, the largest index
		for which ax[index]>=x, except index=0 if ax[0]<x

Notes:
This function is designed to be particularly efficient when called
repeatedly for slightly changing x values; in such cases, the index 
returned from one call should be used in the next.

Author:  Dave Hale, Colorado School of Mines, 12/25/89