Extract constants GUTTER_WIDTH/HEIGHT to layout-constants

The same values are now used by reference in the two modules
'plot' and 'layout'.
Move Visualization/Data selector on top of visualization

I noticed that the buttons were hard to push when testing
the tool on a mobile phone were graphics tend to overlap.
I moved the buttons after the graphics in the DOM, to ensure
that they be displayed on top.
Restore background of button to transparent, not white

The buttons for the selection of Visualization / Raw Data
used to display on a white background, hiding the underlying
visualization in case of overlap. I restored the default
background color instead, transparent.
Rename parameter 'svg' to 'group'
Remove intermediate 'g' element wrapping each tile
Highlight author circles on hover

The circle is highlighted when the mouse cursor hovers either
over the tile circle or the adjacent tile bounding box.

The hover effect is a bit slow in Firefox, not as responsive
as in Safari. I will check whether applying the effect with
a descendent selector when a grouping tile is hovered makes
any difference.
Wrap each tile in a 'g' element to capture 'hover' for CSS styles

The performance is visibly better in Firefox, while still using
the adjacent sibling selector, with a number of adjacent siblings
limited to 1 within the 'g' element of each tile.
Simplify selector for hover effect on each tile

I replace two selectors, including an uncommon adjacent sibling
selector, with a single child selector.
Move bounding box from bounding box 'rect' back to tile 'g'
Add styles and sample markup for the card with author details
Hide the author card initially
Inline three functions which return a fixed CSS class name
Remove unused class 'tile-box'
Complete implementation of showElement / hideElement

The implementation is based on two new utility methods, addClass()
and removeClass(), which respectively add and remove one CSS class
from an element, using HTML5 classList property if available, or
falling back to a string manipulation of the className property.
Show / hide author card when over / out of a tile
Revert unwanted changes introduced in markup together with the card

Revert "Add styles and sample markup for the card with author details"
This reverts partially commit ab3746376e782b6ab0aa38d7ed3e597f783ef3e7.

Conflicts:
	index.html
Define mouseover/out handlers on whole tile instead of the bounding box

The display was previously interrupted when the mouse moved from
the bounding box rect to the circle on top.
Add TODO comments about use of CSS3 transitions for fadeIn/fadeOut
Use "total-authors" event instead of "authors" + countData()
Remove empty line