pushed cometTailDirFunctions

Pushed branch */cometTailDirFunctions

Functions added to Comet class:

Vec3d plasmaTailDirection();
- anti-sunward direction, radially off by 3 degrees

Vec3d velocityOfComet();
- tangent to comet's orbit; current velocity direction
- for dust tail (which curves with orbit)

Vec3d dustEjectionVelocity();
- for dust tail's anti-sunward direction vector
- to be used with velocityOfComet() to get final dust tail direction

float tailLength();
- Length of dust and plasma tails (equation does not distinguish)

void plasmaTail();
- to draw plasma tail
- note: not working yet. Need to figure out how to make it display on Stellarium and which function in Planet's draw function to put in.

void drawComet();
- virtual function in Planet
- to draw the comet tails in addition to what Planet does