Include an attribute for result generation timestamp

[ASL-rmarshall]

The date/time of result generation (or analysis result program execution) is usually needed for auditing purposes and may be included in, for example, display footers.

This also raises the question of whether it should be possible represent results from multiple executions of the analysis result programs. If this is necessary (e.g., so that it's possible to compare results from separate executions of the the same version of the analysis), then it may be necessary to introduce to introduce something like "result sets" as timestamped sets of results, e.g.:

[ASL-rmarshall]

This was discussed on 2024-03-13 and it was agreed that this change should be considered for a future version of ARS because additional consideration is required. For example, while the proposal above suggests creating resultSets within the Analysis class, it's unclear whether it might be better to allow "result sets" to include results from more than one analysis. One option for consideration could be to move results out of analyses (e.g., to be a separate component of the reporting event itself). These results would need to retain a reference to the analysis that generated them (e.g., via analysisId), but could possibly be grouped in different ways.