Initial connections with stoichiometry

• Initial connections with stoichiometry.
• More analyses of discontinuities.
• Rocket analysis and simulation.
• Changes in notation.
• More exercises.

v0.3

The core part for Mechanics is ready.

Still missing:

• Electromagnetics
• More connections between Mechanics, Chemistry, Electromagnetics
• Teacher's guide
• Additional motivation for the textbook
• More exercises