Reimplement the discrete distribution functions using lookup

[David Bateman]

For information as a follow-up to the bug report

https://savannah.gnu.org/bugs/?29545

I reimplemented the discrete_cdf, discrete_pdf and discrete_inv function 
that are also used in the empirical_* and unid* functions. A short 
benchmark script is

vmax = 1000;
v = 1:vmax;
p = rand(1,vmax);
p = p / sum(p);
for n = [100,500,1000,2000]
x = ceil(vmax*rand(1,n));
y = rand(1,n);
tic
discrete_cdf(x,v,p);
t1 = toc;
tic
discrete_pdf(x,v,p);
t2 = toc;
tic
q = discrete_inv(y,v,p);
t3 = toc;
fprintf ("N %i: CDF %f s, PDF %f s, INV %f s\n", n, t1, t2, t3);
endfor

which when run with a recent tip gives

N 100: CDF 0.033175 s, PDF 0.018628 s, INV 0.022826 s
N 500: CDF 0.064294 s, PDF 0.064881 s, INV 0.073642 s
N 1000: CDF 0.128736 s, PDF 0.130771 s, INV 0.153936 s
N 2000: CDF 0.273697 s, PDF 0.274693 s, INV 0.290557 s

and after the changeset I just pushed

N 100: CDF 0.005032 s, PDF 0.005660 s, INV 0.005447 s
N 500: CDF 0.005283 s, PDF 0.004369 s, INV 0.005893 s
N 1000: CDF 0.005880 s, PDF 0.006766 s, INV 0.006027 s
N 2000: CDF 0.006776 s, PDF 0.006126 s, INV 0.005350 s

There are probably other statistics functions that could profit from the 
same treatment

Cheers
David