Img data pixel sizes units

[will-moore]

This is a continuation of discussion at #142 (since that branch was incorrectly named and then deleted).

Latest commit now adds additional valueX and valueY attributes to the imgData json instead of overwriting existing pixel_size x and y. So now the json looks like

pixel_size: {
    symbolX: "mm",
    unitY: "MILLIMETER",
    unitX: "MILLIMETER",
    y: 9800,
    x: 9800,
    symbolY: "mm",
    z: 9800,
    valueY: 9.8,
    valueX: 9.8
},

cc @chris-allan.

[chris-allan]

Looks great from a technical perspective. Will let someone else comment on how things are working in the user interface.

[pwalczysko]

Did analogical comparisons as described in #142 (comment), except that left out the Insight part. All looks good between webclient and Figure for real images in nm, dm, mm.
All good here by me.