Add option to specify amplitude in ångströms in addition to steps in AFMulator

[NikoOinonen]

The definition of the scan amplitude is kind of confusing at the moment as described in #336 (comment)

Basically, instead of specifying the number of steps, specify the amplitude in Ångströms.

Some of the other parameters have kind of confusing names as well:

• The pixPerAngstrome could be renamed to something like ff_grid_density to be more descriptive. Maybe it would make even more sense to specify the grid step instead and have something like ff_grid_step.
• The rho and rho_delta parameters for the tip densities behave differently depending on which force field model is used. We could instead have something like tip_density_es and tip_density_fdbm to clearly indicate what they are used for.

[NikoOinonen]

@ProkopHapala Do you have any opinion on this?