`Group.toBound`'s return type wrongly marked as `Group`

[suguruwataru]

As a convenience method for creating a Bound from a group, this wrong type annotation makes it less convenient... Type check wrongly complains about passing a Group where a Bound is expected and we don't get completion on Bound's members.

pts/src/Pt.ts

Lines 946 to 947 in 47de4b7

 toBound():Group { 
   return Bound.fromGroup( this );