Enforce required elements

The "required" field of the spec accepts 5 different values. They're not documented very prominently (#496 ), but there's a definition for them here:

https://github.com/ignitionrobotics/sdformat/blob/12e77821af7c2810c4d9d532ee8cb833cbb8e318/include/sdf/Element.hh#L109-L113

### Current enforcement

Looking through the code, I found some places that make use of them:

1. Deprecated elements (`-1`) issue errors:

https://github.com/ignitionrobotics/sdformat/blob/12e77821af7c2810c4d9d532ee8cb833cbb8e318/src/parser.cc#L1491-L1495

1. Elements that must have exactly `1` or must have at least one `+` issue errors:

https://github.com/ignitionrobotics/sdformat/blob/12e77821af7c2810c4d9d532ee8cb833cbb8e318/src/parser.cc#L1504-L1509
https://github.com/ignitionrobotics/sdformat/blob/12e77821af7c2810c4d9d532ee8cb833cbb8e318/src/parser.cc#L1903

But that 's only part of it.

### Missing enforcement

Should these also cause errors (they're all currently valid according to `ign sdf -k`)?

---

More than one element when it should have exactly one (`1`):

SDF

```

a
b

```

---

The difference between `0` and `*` should be clarified in the documentation and also in the enforcement. I assume that the existence of `*` implies that `0` means "either none or one". In that case, this should be an error:

SDF

```

```

Start with the required-value definitions in include/sdf/Element.hh and the existing checks in src/parser.cc around the cited lines. Reproduce the examples with `ign sdf -k`, then inspect the parser tests and documentation related to required elements. Done means the intended `1`, `0`, and `*` semantics are documented and consistently enforced with regression coverage.

Written by the indexing model from the issue text.