New surface integrators 'path_trace' and 'null_integ'

Two new surface integrators have been added:

- 'patch_trace' performs path tracing
  (to be refined in future versions)

- 'null_integ' simply returns Spec.white for any ray
  (usefull for performance testing, etc.)
SPDF matrices and film patches

The sub-module Spec.SpecMat provides matrices of Spec.t values, where the
actual values are stored "flat" in a float32 - BigArray. Therefore, the
memory layout is more compact and marhalling/unmarshalling more efficient.

A film is a matrix of SPDF values together with a corresponding matrix
of weights. The Film signature in Module Image has been changes such that
also fractions of films (defined by a rectangle of the image plane) can
be represented. Film patches are now used to communicate results back from
a worker process to the main process. This way the communication overhead
only depends on the reconstruction filter extend that defines the minimal
patch size.
Generation of film patches reworked (module Image).
The following changes have been merged into the master branch:

* Film patches as a means to communicate back results from worker processes
  to the main process, instead of lists of samples.

* First version of a path tracing surface integrator.
removed debuging code
OpenEXR support via the EXRIO tool