How to make your own custom filter?

As mentioned earlier, a filter must take at least one input et retrieve at least one output. In between, the method .update() must be called. The only exception to that are io filters which are opening or writing from/to a file or an HTML5 canvas.
Most of the time, regular filters output one a PixpipeContainer, but in certain cases we can imagine having a filter that perform statistics and outputs a report.