Fix types of get_mapgen_setting_noiseparams

[appgurueu]

Bugfix. Closes #11962, ready for review. Mainmenu noiseparam setting editing still works. @srifqi

Regarding possible breakage:

• The documentation mentions numbers and not strings, so the old behavior was clearly a bug nobody should have relied on;
• When passing params to setters both numbers and strings were, are and continue to be supported (by means of tonumber);
• If somebody tonumbered the strings, that is a no-op for numbers and works as expected there;
• If somebody performed arithmetic or called math funcs on the strings, relying on the implicit conversion Lua does, that will obviously work with numbers too;
• And even most (all?) string operations will, due to the same implicit string-number conversion, just stringify the number;

The only thing this would break would be actual typechecks for a buggy type or trying to index the number (something like noiseparams.something:sub(...), but I suppose that hardly matters. Should it be found in the wild, it's definitely utter swinecode and on the mod authors, who should be urged to upgrade.

Also note that prior to 5.0, said getter would return numbers.

[SmallJoker]

LGTM

[gaelysam]

I'm coming too late but %g string serialization needs to be applied in the previous function get_current_np_group too

Causes rounding issues in the dialogs

(I don't know how to make a comment outside the scope of the changes)