SMALL API CHANGE: is_valid->isvalid, plus some pretty print changes, etc


* BREAKING API change: is_valid() changed to isvalid
* Implements a __len__ method on FeatureVector
* Prettier prints out FeatureWeight report info
* Fix typo in error message 
* New data attributes on class FeatureSet:
    * name
    * contiguous_imagenames_list (contiguous_imagenames_list:imagenames_list::data_matrix:data_list)
* Better checking for interpolation_coefficients are present on FeatureSet instances
* Added quiet argument to BaseGraph.__init__