Remove the hard-coded RocketParts density.

Having 1t/u for the density of RocketParts is very harsh when 1m^3 can hold
around 200u of anything else. This change doesn't actually change the
density of rocket parts, but it does make it so changing the density will
cause the right amount of rocket parts to be used when building a ship.
Remove the hard-coded SolidFuel density.

While I don't consider it to be likely, Squad might change the density of
SolidFuel. This future-proofs EL against such a thing.
Sanitize the resource densities.

This paves the way for better inter-mod compatibility (eg, MFS).

The calculations assume 200 units/m^3 (5l/u)
Ore has been set to 5.5t/m^3 or 0.0275t/u.
Metal has been set to 7.8t/m^3 or 0.039t/u.
RocketParts has been set to 0.5t/m^3 or 0.0025t/u.

This makes shipping RocketParts very cumbersome, but Metal fairly
efficient. Storage is a bit of a problem, too, but a Roxomax Jumbo-64 can
still hold 16t of parts, and a large spherical tank can hold 96t.
Update the deposit quantities to suit the new ore density.

Just to save on hair loss, the values have been multiplied by 40 rather
than 36.3636...
Merge pull request #35 from taniwha-qf/resource-density

Resource density