[BUG] Can no longer use float increments for `n_range` argument

[zoj613]

Describe the bug
It seems like PR #120 (particularly this commit: 490e676 (#120) changed the behaviour of the package. I recently ran benchmark plots for a package I develop where I use float values for n_range and suddenly got unexpected behavior: The program froze! Upon further investigation I realized that the values passed to n_range were being truncated and one of them ended up being 0 (truncated from 0.1). The underlying function that is supposed to accept the value of n_range caused the interpreter to crash because 0 is forbidden given the setting of the script.

To Reproduce
Just set n_range=[0.1, 0.4, 1.5] or any float, it will get truncated to ints.

Diagnose
I may ask you to cut and paste the output of the following command.

pip freeze | grep perfplot

Ouput: perfplot==0.9.9

It seems like this change introduced by #120 is breaking. Would it be too much to ask for re-introducing this functionality? For reference, here is the script that I use in order to generate the plots and the x-axis using float values in increments of 0.4 from 0.1 to 40. From v0.9.9 on wards, This script does not work.

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