logarithmic growth curves

[jacopogrecodalceo]

Could it be interesting to add exponential and logarithmic curve?
Thank you for your work!

[johannphilippe]

I added an approximation on logarithmic curve, called hc.logarithmic_curve (Lua) and hc_logarithmic_curve (Csound).
Same for exponential.
It takes no argument and is to be considered as an first approximation. We should add a more flexible logarithmic and exponential curve later
Since logarithmic and exponential functions have no limit, we should find a convenient way for users to decide which part of the curve they need.