Want to remove `undefined` on tooltips & addressing overlaps between drivers

[vstevensf]

If a driver file does not include one of the optional attributes (according to documentation), the display shows "undefined", rather than a blank. More of a style issue, but of note if the user doesn't want "undefined" displayed. Otherwise, the user must include these attributes (columns) explicitly within a given driver file, with blank/empty values.

[vstevensf]

Here is an example where, if all columns (including optional) are filled in, even with a blank, there will be no "undefined" value in the tooltip when hovering over a driver

[vstevensf]

Lastly, whenever there are driver mutations close to one another (or in very rare cases, basically the same position), there is high overlap and one isn't able to accurately use the tooltip in the circular view to view metadata. Would be helpful in these cases to include the name of the gene in question in the tooltip (redundant but useful), or handle the overlap in a different manner that may not necessitate the user to zoom in within the linear view. Image below :)

[sehilyi]

Issues from the first two comments are addressed in the latest commit (a24dd36).

Regarding the last comment about the visual overlaps, perhaps we can spread out the near-by genes to address the overlaps.