Changed to use autosummary for the to_ attributes.
Added documentation and doctests for inlink functions.

Added
-----
* Added documentation and doctests for:
  - active_links
  - setup_active_inlink_matrix
Added files for structured and rectilinear grids.
Added structured_quad subpackage of grid.

Added
-----
* New subpackage, structured_quad, for general structured grids of quadrilaterals.
* New module, links, for working with link of structured grids of quadrilaterals.
Moved rectilinear and structured modules in structured_quad.
Added function to get total number of links.
Added modules for cells and nodes of structured quad grids.
Moved rectilinear and structured modules into structured_quad.
Added function to get corner nodes.
Added module for faces of a structured quad grid.
Added iterator functions for perimeter nodes.
Fixed number_of_cells to return correct value.
Added number_of_core_nodes function.
Added __init__.py to make structured_grid a subpackage.
Added node_ids and status_with_perimeter_as_boundary.
Added shape_of_cells and node_id_at_cells.
Added cell_ids function.
Functions to count number of in/out links per node.

Added
-----
New functions:
  * number_of_in_links_per_node
  * number_of_out_links_per_node
Added functions to get in/out links per node in compressed form.
Added functions to get node ids at link start/end.