lib_dv delta-v calculation and decouplers/engine plates

[ElWanderer]

The delta-v calculation deliberately stops traversing through the parts list each time it hits a "decoupler".

BUT: if the decoupler has crossfeed enabled, we should actually carry-on through it to the parts beyond. This is especially true of the engine plates (DLC content?) as those are decouplers that sit between the engines and fuel tanks.

Also, not that these necessarily belongs here:

• can we tell if fuel is enabled/disabled on a tank part?
• can we take into account the ratio of LF/OX?

[ElWanderer]

Decouplers now block parts from being considered by the delta-v code if both of the following are true:

• they have the appropriate decouple event - this considers engine plates to be blocking until they are staged
• they don't have the "disable crossfeed" event - this implies they currently have crossfeed enabled

Also, it looks like enabling/disabling individual resources through the part action window does feed directly into the data kOS gives us. Worth checking that further in future, though.

As it stands, we simply add up the fuel mass in each part. In order to check ratios, we would need to keep track of the mass of each type. That's probably something to raise separately.

[ElWanderer]

can we tell if fuel is enabled/disabled on a tank part?

When adding up the fuel mass from parts, each resource can be TOGGLEABLE, in which case we should check whether it is ENABLED

http://ksp-kos.github.io/KOS_DOC/structures/vessels/resource.html#attribute:RESOURCE:ENABLED

(this may be worth raising as a separate issue)