Adding zoom factor to initialization options

[ivan-katkov]

Currently, the default zooming step is too large (for my usecase) making it difficult to fine tuning the size of the view region. However, for viewing very large fields, the current zoom factor can be more appropriate. The ability to adjust the zoom factor/step could address this.

[bmatthieu3]

@ivan-katkov - I am not sure to fully understand because you can already set the fov in degrees at intialization.
Do you mean that the zooming is too fast (i.e. zoom step is too big) when zooming with a big fov ? Can you add more precision, thank you.

[tboch]

might be linked to #202