make samplesmixin more generic and add some useful transformers

bob/pipelines/__init__.py

bob/pipelines/mixins.py

bob/pipelines/sample.py

bob/pipelines/test/test_mixins.py

bob/pipelines/transformers/__init__.py

bob/pipelines/transformers/linearize.py

bob/pipelines/transformers/pca.py

bob/pipelines/utils.py