Implement date-class date precision

[japonicus]

Some low-level support exists for dates that are expressed only in terms of a date-class range, but this functionality is currently unused.

The main benefit of full implementation of date-class precision would be to make it explicitly clear to data users when a broad date estimate had been applied.

There are several types of possible usage:

1 Dates where a range fits an exact date-class (start date and end-date match the same dateclass)
2 Date ranges with the start date clamped to a dateclass but a non-standard end-date - a convenience to trigger broadly correct mapping of a vague record (spanning 1 or more dateclasses)
3 Date ranges that begin and end on a date-class boundary, but which span multiple classes. (this will be relatively rare unless a 1950-1969 class is inserted to split 1930-1969, in which case there will be thousands of records)

The greatest difficulty is devising an editing user-interface that captures these concepts satisfactorily.

The editor user interface could be modified as follows:

the date range menu could be expanded to consist of

<select><option value="a" selected="">single date</option><option value="r">date range</option><option value="l">before</option>
    <option value="0">pre-1930</option>
    <option value="1a">1930-1949</option>
    <option value="1b">1950-1969</option>
    <option value="2">1970-1986</option>
    <option value="3">1987-1999</option>
    <option value="4">2000-2009</option>
    <option value="5">2010-</option>
    <option value="dc">other dateclass range</option>
</select>

which may be too unwieldy

or

<select><option value="a" selected="">single date</option><option value="r">date range</option><option value="l">before</option>
    <option value="dc">dateclass range</option>
</select>

where dateclass range is combined with range date options consisting of two menus (for from and to)

<select>
<option value="0">pre-1930</option>
<option value="1">1930</option>
<option value="1b">1950</option>
<option value="2">1970</option>
<option value="3">1987</option>
<option value="4">2000</option>
<option value="5">2010</option>
</select>

<select>
<option value="0">pre-1930</option>
<option value="1a">1949</option>
<option value="1b">1969</option>
<option value="2">1986</option>
<option value="3">1999</option>
<option value="4">2009</option>
<option value="5">present</option>
</select>

The 'latest date' option could be modified to encourage recorders to specify a start date-class from a menu (whereas the implicit current behaviour is to map open-start records as DC 0, which is less than satisfactory)