Add support for Size Computation Expression for edges

[mt-empty]

• I have checked that this feature has not yet been suggested by someone else.

Expected behavior

Size Computation Expression is currently ignored and every edge has a size of 1.

Cuttings backs

...

Potential side effects

...

Describe alternatives you've considered

Editing the CSS.

[pcdavid]

PR #409 seems to work, but still needs some automated tests before it can be merged.

With a simple sizeComputationExpression="aql:self.name.size()" on a reference-based edge I get:

[mt-empty]

Just to be clear, I was referring to Sirius web edges. stroke-width doesn't map sizeComputationExpression

[pcdavid]

I undestand, the screenshot in my previous comment is from a Sirius Web modeler. Here the stroke-width is 6 because that is the length of "Engine", i.e. the result of aql:self.name.size().