bugfix for tool material radiobuttons

[spanner888]

uncommented get_surface_speed() so that current value of tool Material
radio buttons used in calculations.

Without this, changes to Tool material ignored.

reported by FC forum user jescombe
https://forum.freecadweb.org/viewtopic.php?f=15&t=59856&start=40

[dubstar-04]

Thanks for this! I will have a look at it asap!

[dubstar-04]

This line of code was commented out when splitting the ui and the feeds and speeds class as the user can override the surface speed on the form.

I have included a fix for this in another PR. #20

I would appreciate it if you could have a look.