Give up placing theta-grid in constant memory for simplicity
Use shorter names for local variables
Use single character names for local indices
No need to keep the parameters "Rd" and "Rf"

We are refitting everything in the paper.  The two parameters "Rd" and
"Rf" are outdated.  Use their inverses "td" and "tf" instead.
Use a portable way to initialize structs to zero
Set d.I[] = d.tau[] = 0 for s.r < c.r[0], instead of extrapolating
Find i and I so that c.r[i] <= s.r < c.r[I]; compute weight f
Improve the i-I finding algorithm
Find j and J; compute weight g; fix bug in i-I finding algorithm
Compute k, K, and h for interpolation
Interpolate the grid properties
Rearrange some formulae to (hopefully) improve performance
Implement field interpolations
Fix bug: correctly compute the weight
Merge branch 'interpolate' into develop