[PR] Convenient spacing configuration

[uruuru]

Efforts in the direction of #105.

The new class ElkSpacings is intended to be used by layout algorithm-integrators to configure a graph with spacing values before the graph is passed to the layout algorithm itself. It supports the spacing options provided by CoreOptions.

The LayeredSpacings class adjusts and extends the functionality of ElkSpacings by what layered offers in terms of additional spacing options (in particular the between layers options).

Furthermore, there's a new layout option in Layered.melk: spacing.baseValue. It can be used to alter the overall "spaciousness" of a drawing. In other words, in can in-/decrease all spacing values. If one desires to fine-tune individual spacing option values, the LayeredSpacings class would have to be used manually as described for ElkSpacings above.

@le-cds I'm interested in your opinion regarding the actual integration into layered's ElkGraphImporter (see the code comment for a rationale).

Documenting the usage of the new classes as part of the website shall be done as part of #335.

[le-cds]

Wow, that's quite a bit to take in, thanks for all the effort! I find it hard to really evaluate it without using it, but just from going over the code, it does make sense.

Using this also solves our problems with layout option defaults when it comes to spacings, right?

[uruuru]

Using this also solves our problems with layout option defaults when it comes to spacings, right?

I'm not sure I know what you mean.

[le-cds]

I'm not sure I know what you mean.

I'm basically referring to #104. This mechanism applies concrete spacing values to all elements, so that the node and label placement and size code won't have to rely on wrong, non-algorithm-specific default values?

[uruuru]

Sounds like it 😲.

I'll see if I can add a test to verify this, though.

[uruuru]

It does - I added a straightforward test. Note however, that the application of concrete spacing values only happens if the baseValue property is used (and if the configured base value is different from the default value).

We could discuss whether we want to execute this kind of mechanism by default for all options that are used within common layouter code. However, as currently all default values match between core and layered, there's no real need to do so.

[le-cds]

We could discuss whether we want to execute this kind of mechanism by default for all options that are used within common layouter code. However, as currently all default values match between core and layered, there's no real need to do so.

I guess we could defer that to when this really becomes a problem.