SolidModel, postrender_ops() issue

The following `postrender_ops` shows `Error : Difference failed - BOPAlgo_AlertTooFewArguments ` error.
```
(
"l1_gnd",
SolidModels.union_geom!,
("l1_gnd", "bump_surface",2,2),
),
(
"l2_gnd",
SolidModels.difference_geom!,
("l2_gnd", "l1_gnd",2,2),
:remove_object => true,
:remove_tool => false
)
```
However, after changing the group name of `union_geom!` from "l1_gnd" to something else, the error goes away.
```
(
"foo",
SolidModels.union_geom!,
("l1_gnd", "bump_surface",2,2),
),
(
"l2_gnd",
SolidModels.difference_geom!,
("l2_gnd", "foo",2,2),
:remove_object => true,
:remove_tool => false
)
```

Start by reproducing the reported postrender_ops sequence using SolidModels.union_geom! and SolidModels.difference_geom!, comparing the "l1_gnd" and "foo" group-name cases. Read the implementations of those operations and determine why reusing "l1_gnd" produces BOPAlgo_AlertTooFewArguments; done means the original group names no longer trigger the error while the geometry operations still produce the expected result.

Written by the indexing model from the issue text.