Add some integration + example

[ryanlevy]

This is the beginning of adding integration and integral equations

[ryanlevy]

Quick notes:

• Major cleanup and explanation of whats going on in the example
• Renamed function operator(f) to function operator_proj(f) (I dont like mixing variable names with function names too much)
• swapped from show(plot) to display(plot) which inserts a new line after the plot, making things look nicer
• Tweaked some of the hyperparams for the integration example to run faster with a very small increase in error

[JoeyT1994]

I think we should add a test for this. Maybe just a file called test_integration.jl. The first two experiments of the example can be in those tests. Then the example can be solely the Fredholm solver and we can call it examples/fredholm_solver.jl