pygr: fix WC calculation on log scaled axes

Fixed in CoordConverter getWC:
* linearise min, max scale - log10(min), log10(max)
* calculate exponent e
* WC = 10 ** e
qtgr: Change mouse wheel zoom behavior

Zoom in steps of 10%
* Most mouse types work in steps of 15 degrees
* If the mouse wheel has finer-resolution and send steps less than 15 deg
  zoom steps less than 10% proportional to steps in degree will be used.