Use fixed origin for BEAST BDSKY model if tradition has a fixed composition date

[jjmccollum]

Suppose the work whose tradition is collated in the TEI XML input has a known date specified with a date element with a when attribute, as follows:

<bibl>
    <title>Title of the work</title>
    <date when="50"/>
</bibl>

Currently, this maps to a Uniform distribution whose lower and upper bounds match, which raises an exception in BEAST 2:

<distribution spec="Prior" id="originPrior" x="@origin">
    <distr spec="beast.base.inference.distribution.Uniform" lower="1974" upper="1974"/>
</distribution>

If the date is known, then this distribution element is unncessary in the BEAST XML output. We can simply add estimate="false" to the origin element:

<origin spec="parameter.RealParameter" id="origin" lower="1974" upper="1974" value="1974" estimate="false"/>

Likewise, the operator element with id="originScaler" and the log element with idref="origin" can be excluded in this case.

These changes should be made in beast_template.xml and the to_beast method in collation.py.