Handling more than 3 decimals entered for group order granularity or < 0.0005

[twothreenine]

The group order granularity is saved with a scale of 3 decimals.
However, it is possible to enter the following numbers:

entered	saved
0	0.000
0.0001	0.000
0.98765	0.988

A group order granularity of 0.000 currently seems to behave like 1.000, except that the -/+ buttons clear the input field.

Suggestions:

• Add a warning that the number will be rounded to 3 decimals (show only if more than 3 decimals entered?)
• For numbers < 0.0005,
  • either forbid them and show an error message
  • or show a warning and replace them with 0.001 as it is the smallest amount that can be saved (could be neat to allow entering 0 instead of having to type 0.001 for many articles, so I'd prefer that)