chore: update round_step_size to use quantize

[pcriadoperez]

Ran tests with old code and new code and gave the same result.
There was one exception where old code if step size was 1.0 would round like a step size of 1 while current implementations step size 1.0 rounds like 0.1

Fix #1496

[coveralls]

coverage: 67.986%. remained the same
when pulling 2e27c2a on pcriadoperez:round-step-size
into 10f3986 on sammchardy:master.

[christophlins]

Please note that accepting float might lead to undesired results:

round_step_size(0.09999999999999999, 0.1)
Out[11]: 0.0

round_step_size(0.09999999999999999999, 0.1)
Out[12]: 0.1

If the type of the argument is float, add an eps (machine precision) prior to rounding.