mathematical formulas in doc

Added a custom HTML generator for OCamldoc in order to include
mathematical formulas in the documentation (to be rendered with MathJax).
Added postprocessing of generated HTML files with a sed-script for the 'html'
rule in the Makefile. The sed-script adds HTML script-tags to setup MathJax
for rendering the math content of a page.
Reimplementation of the Bbox module in C

The Bbox module has been reimplemented in C (reasons detailed in doc).
Moreover, function Bbox.bintersect has been added.
C-implementation for generating permutations of integers

The generation of integer permutations is now performed in a
C-implemented function (cf. file rngc.c and module Rng).

A new function get_array has been added to module Rng that returns
an array of random values. The function is not used, yet.
minor corrections in module Bbox
Minor changes in module Bbox after testing.
Minor changes in module Rng after testing.