Resolve "Time Integration Solvers"

Core:

• Adds FixedStepArithmeticNumber for integer-based floating point arithmetic
• Adds methods for safe comparison of floating-point numbers
• Adds implementations for least common multiple and greatest common divisor

Ode:

• Adds interfaces to describe systems based on ODEs and time-discrete parts
• Adds solvers for time-integration based solving of such systems.

Closes #16 (closed)