Adds a netloc_reader_static, and improves documentation.

     * Add a netloc_reader_static for networks that the users wishes to describe by hand using a JSON format defined for this reader.
     * Add a JSON Schema file and validation script for the netloc_reader_static
     * Create an example input file for netloc_reader_static
     * Add some checks for NULL data inside the library
     * Issue a warning and error out if the user tries to replace a valid node in the data collection set.
     * If a node is already in the data collection and append_edge_to_node is called then the cached information is updated properly.
     * Document the netloc_reader_static
     * Document the data collection API
     * Update gitignore