Refactor simIncludeGravBody

[juan-g-bonilla]

Describe your use case

Current syntax for adding gravity to the simulation is slightly clunky, and we could do better. I propose the following changes:

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scObject.gravField.gravBodies = spacecraft.GravBodyVector(list(gravFactory.gravBodies.values()))

to

gravFactory.addBodiesTo(scObject)

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from Basilisk import __path__
bskPath = __path__[0]
gravFactory.createSpiceInterface(bskPath +'/supportData/EphemerisData/', timeInitString, epochInMsg=True)
gravFactory.spiceObject.zeroBase = 'Earth'

to

spiceObject = gravFactory.createSpiceInterface(time=timeInitString, epochInMsg=True)
spiceObject.zeroBase = 'Earth'

Explanation: Have bskPath +'/supportData/EphemerisData/' be the default path for createSpiceInterface (with the ability to still set your own path). Also, return spiceObject (keep it accesible through gravFactory.spiceObject too).

---

from Basilisk import __path__
bskPath = __path__[0]
asteroid.useSphericalHarmonicsGravityModel(bskPath + '/supportData/LocalGravData/VESTA20H.txt', 20)

to

asteroid.useSphericalHarmonicsGravityModel("%BSK_PATH%/supportData/LocalGravData/VESTA20H.txt", nSpherHarm)

Explanation: Make %BSK_PATH% a stand-in for Basilisk.__path__[0].

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All these changes can be implemented while keeping the old syntax functional.

Additional context
Some of these changes assume that the syntax allowed by #331 is implemented.