Multi fuel preparation part 2

[jkirk5]

Summary

This completely overhauls how fuel is calculated by Aviary and rips out many undesirable assumptions.

• Moves GASP fuel capacity calcs from mass to energy subsystem
• Update all fuel calculations to no longer assume mass delta = fuel burned (now fuel burn is integrated over the mission and summed in post-mission)
• Fixes to fuel summation test - it was using the wrong inputs and therefore never testing the full fuel summation component.

While updating tests and checking this conversion worked, I found several issues in the tests and updated them:

• Reserve missions were not being fully tested - new benchmark added
  • Updated reserve missions to allow for discontinuous trajectories (mach & alt are not linked to the first reserve phase if user sets an initial value in phase_info for that variable), working for energy phase only right now
• Streamlined how subsystems are loaded for tests by combining get_default_premission_subsystems, get_default_mission_subsystems, and get_geom_and_mass_subsystems into a single util function - this makes keeping tests on top of any additions/removals/changes to the core subsystem lineup much easier
• Found some tests were not actually checking partials (comparing cs or fd with itself), fixed this and updated broken partials uncovered by this change. I suspect there are more tests in this situation we haven't caught yet

Backwards-incompatible changes

Mission.RESERVE_FUEL_MARGIN was changed to be specified as a direct multiplicative value to be applied, rather than as percentage points (e.g. 0.2 rather than 20). This was done for consistency, as every other Aviary variable that deals with fractions/percentages works this way. A deprecation check was added to the preprocessor to alert the user of this.