sync latest graph ref master
update get_subplots() docstring
remove old get_subplots() code
add Exception if rows or/and columns are not integers
make h/v spacing depend on the number of rows/columns:

- breaks backwards compatibility
- horz = 0.2 / columns (same as old if rows==2)
- vert = 0.3 / rows (same as old if columns==2)
initialize specs list of lists:

- subplot domain computation are made using specs whether or not
  it is a supplied argument
compute some common distances
loop through specs:

- use tracers (x and y) to keep track of position
  from subplot to subplot
- cases where colspan > 1 or rowspan > 1 require more attention
- use _get_shared() to get the shared axis (if any)
- use _add_domain() to paste result into fig object
- use _fill_grid() to generate the str repr of the subplot grid
add _get_shared()
add _add_domain() and _add_domain_is3D()
add _fill_grid()
initialize looping variables
(oops) get_shared -> _get_shared
add print_grid
modify computed domain in existing tests (because of rounding errors)
modify arguments in existing tests:

- because of new horizontal_spacing and vertical_spacing defaults
test galore git diff --cached !