Add some developer documentation for widely used structs/methods

[BrunoMartinGym]

First, let me start by saying that this application is amazing. A thing that could make it even more useful to new developers would be to add some documentation directly in the code. For example:

1. KinFuParams: This struct contains lots of parameter for which the impact of modifying the values is not documented. Adding some guidelines/rule of thumb for the use of these parameters would be welcome to new users. For example: What parameters should I pay more attention if I want to scan small objects, objects with sharp features, etc?

2. What is the meaning of the different flag values for the KinFu::renderImage method?