Parametrized miter box

Parametrized miter box with two versions of channel labeling that can be turned off.

Attached .stl files are with following parameters:
```
box_width = 40;
box_length = 100;
box_height = 30;

box_floor_heigth = 5;
box_wall_width = 5;

channels = [30, 45, 90];
channel_width = 1.5;

add_channels_description = true;
text_height = 4;
```