PyResult: wrapper for validation results

- wrapper for nix::valid::Result
- wrapper for nix::valid::Message
PyResult: docstrings
PyFile: bindings for validate
Merge pull request #107 from stoewer/nixpy-34-validation

Bindings for file validation
quick fix of broken build due to new overload of hasProperty in nix
Merge pull request #108 from jgrewe/master

quick fix of broken build due to new overload of hasProperty in nix

LGTM
bindings for sampleDimension helpers
bindings for added sampled dimension methods, added docstrings
changed tutorial code to use the sampled dimension methods
Merge pull request #109 from jgrewe/dimensions

Bindings fuer SampledDimension methods

Nice!
section: turn "S" object into transparent wrapper
[test] test transparent access for nix.S
Merge pull request #110 from gicmo/S_wrapper

Turn nix.S into a transparent section wrapper

LGTM
added tests for dimension methods
fixed copy and paste bug in index_of method
Merge pull request #111 from jgrewe/dimensions

Dimensions added tests and fixed a copy, paste bug

LGTM
[tutorial] changed example code to use the retrieve_feature_data method
[tutorial] add section for retrieving feature data
[tutorial] add something about sources.
[tutorial] add sources to toc