[BUG] tool tip to show edge values does is too small for contents

[rossduncan]

Describe the bug

In the visualiser, you can hover over an edge labelled with {} to see the value in a tooltip. However the tool tip does not adapt to the display size of the value, and usually the value cannot be seen. The tooltip needs to resize according to the size of the contents.

To Reproduce

Make any graph that has a value on an edge that is (e.g.) pytket circuit. Run the workflow to completion. In the visualiser hover over the edge label. (Should also work with lists.)

Expected behavior

See above.

[johnchildren]

#414 will add indent to json values written to the checkpoints directory which makes them appear in a more readable way in the visualizer.