Allow more than two decimals for deciding the edges

[gerritholl]

When deciding the edges for the visualisation, sift allows only two decimals. However, when data are tightly spaced, a more precise specification would be desirable.

For example, here is the 0.9/0.8 ratio for a specific region, where I would like to define edges between 0.0095 and 0.0115. The closest I can do is between 0.00 and 0.02. It's also too much discretised, but that is the same problem as described in #404.

[ameraner]

A more extreme example is given by @strandgren in EUM Gitlab issue 172: