Add more function and signal generator blocks

[kwehage]

Adds new function blocks:

• Summation block: adds two or more signals together
• Multiplication block: multiplies two or more signals together
• Exponential block: raises a constant to the value of a signal
• Power block: raises a signal to a constant power
• Sigmoid logistic function

Adds new signal generator blocks:

• Uniform random generator
• Gaussian random generator

Closes #10

[kwehage]

@alex-brinkman I think this is ready to go, let me know if you see anything else.

[alex-brinkman]

I agree, nice work!