Ships rebalancing, part 1
allow effective_exhaust_velocity instead of thruster_fuel_use
rebalance hyperspace range/duration after 30% ship mass increase
increase freighter side thrust to be able to land on Earth
adjustments in pirates and delivery/taxi modules for ship rebalancing

* Pirates less interested in ships with no cargo
* More time for delivery and taxi missions
ship rebalancing: interplanetary shuttle, talon, meteor and turtle

Boosting of Interplanetary Shuttle: this ship cannot hyperspace, so it
also does not have reason to minimize mass of fuel. Its big tank give
the ship the best velocity inside the system (except of Talon, which is
ultra-light and pays for it by its small capacity). I also increased the
thrust of its engines to ensure safe journeys even to atmospheres of
biggest gas giants.

Talon: Changing proportions a bit. It still remains fastest ship with
the biggest hyperspace range, which is balanced by very tight capacity -
it is pretty impossible to fit any cargo inside.

Meteor: I gave it a bit worse speed and thrust, so that it won't beat IP
shuttle (which pays for the advantage by not having hyperdrive) nor
Talon (which pays by small capacity). But it still remains fast ship
capable of very good hyperspace range.

Turtle: I increased its cruising speed a bit by reducing hull mass and
increasing fuel mass.
store effectiveExhaustVelocity instead of thrusterFuelUse internally
fix Ship::GetFuelUseRate
clean up Ship fuel use/exhaust velocity accessors
remove dead thrusterFuelUse ShipType member; add comment on GetFuelUseRate
file-internal functions should be static
Merge remote-tracking branch 'jpab/ship-rebalancing'