TEST+DOC in ellipsoid and docs

Added tests for number of actors

modified:   ../dipy/viz/fvtk.py
modified:   ../dipy/viz/tests/test_fvtk.py

Added docs for color_fa normalization of the ellipsoids
and clipping of the csaofs to be positive

modified:   examples/reconst_csa.py
modified:   examples/reconst_dti.py