Add support for column mapping that selects a project default item in batch item importers

[Nicklas Nordborg]

If a property is not mapped to a column in the batch item importers and project default item exists for the property (eg. a sampling protocol) the core will automatically set that property when creating new items.

Note that this is a feature of the core and that the batch importers doesn't care about project default items at all.

But, we would like to change this so that a column mapping could, for example, be an expression that select a project default item. Since there is already support for the JEP parser in column mappings the functionality can be provided by defining a set of new JEP functions. For example:

=default()  // Get the project default value
=default('column') // Get the value from the 'column', but use the project default item for empty values

This would allow the batch importers to use project defaults with both new and existing items.

For background information see: ​http://www.mail-archive.com/basedb-users@lists.sourceforge.net/msg02048.html