documentation updated
bugfixes in Integ.estimate_direct

Two bugs in function Integ.estimate_direct have been fixed:

- using the right DG in the BSDF branch

- weighting factor of 1/2 removed

Several debug logs have been added.
moved old change log from file bsdf.ml to CHANGELOG_OLD
removed debug code from bsdf.ml
removed debug code
BSDF module revisted; Mipmap lookup with scaling factor

The BSDF module has been revisted significantly by introducing an optional
return value of type Bsdf.sample_t for the sample_f methods. A value of None
indicates 'no contribution' and is considerably clearer than the previous
strategy to return dummy values. Moreover, the previous treatment of
specular components was incorrect which has been corrected.

The lookup function of a Mipmap was missing a scaling factor. The factor
is important when combining BxDFs, where the integral over all components
must not exceed one.
moved PDF auxiliary functions from module Aux to Pdfaux
'refine' method for area lights; Cornell box scene
preparations for the introduction of BERTO's new scene description language
new module 'Scene_parser'
implementation of the scene description language

Until now all scenes had been defined in source file 'demo.ml' together
with run-dependent parameters in property files with ending '.inp'.
This practice has now been replaced by specifying scenes in independend
files (ending '.scn') using a simple scene description language.
The language parser is in source file 'scene_parser.ml'.

The old property-file based code has still been kept but will be removed
in near future.