representing "negative evidence"

Representing "negative evidence" or "the absence of evidence one would expect to find" is often important in forming a proof argument. We want the GEDCOM X model to support this concept.

#### Requirements

Documenting negative evidence typically requires the following elements:
* a citation(s) for the body of records searched
* a description of the search (methods, search terms, scope, party conducting the search, date of the search, etc.)

From a modeling perspective, we would like "negative evidence" to look like and use the same mechanisms used to represent "evidence".

#### Proposal

We think that the documentation requirements can be met with a `Document` object. The `text` of the document could be used to described the search—the date of the search, who conducted the search, how the search was carried out, search terms, etc. The `sources` list could contain the source description(s) for the body of records that was searched.

Assuming `Document` as the documentation mechanism for a negative search, how should we associate this `Document` with our hypothesis as "negative evidence" supporting our hypothesis?

In the GEDCOM X model, we use `EvidenceReference` to associate data with a hypothesis as _evidence_.  The `resource` member points to the data being used as _evidence_ and is a required field.  But in the case of "negative evidence", there is no data to point to&mdash;by definition, we have an "absence of data".  It is by _analysis_ that we have determined that there was no data where there ought to have been some.

We would like to propose that we can represent "negative evidence" with an `EvidenceReference` instance with the document describing the negative search as the `analysis` document, and an explicitly missing `resource` reference.

However, as currently defined, an `EvidenceReference` REQUIRES a `resource` reference (explicitly present) and that reference MUST resolve to an object that matches the type of its container (a specialization of `Subject`).  This means that the current constraint would not allow `resource` to be explicitly missing.  Therefore, we would like to modify the constraint to something like the following.

For `resource`:

REQUIRED, except when modeling "negative evidence", in which case it MUST NOT be present.  If provided, MUST resolve to an instance of [`http://gedcomx.org/v1/Subject`].

For `analysis`:

OPTIONAL, except when modeling "negative evidence", in which case it MUST be present.  If provided, MUST resolve to an instance of [`http://gedcomx.org/v1/Document`] of type `http://gedcomx.org/Analysis`

The above changes are include in this pull request.