There is a problem with duplicate elements that needs to be handled.

They can arise if someone uses conjoin to join all files on each
specific processor and then uses epu to join the resulting files into
a single file.  If there is load-balancing, then an element can appear
on multiple processors over time.

Note that the element should only be active on 1 processor at any one
time and its element variables are only valid on the processor on
which it is active.  Because of this, can't do a simple 'last one in'
for the element values; need to pick which instance of the duplicate
element is active at a specific timestep and then use that element's
values for the output value at that step.

Need something like "-remove_duplicate_elements"
                    "-status_var <variable>"
                    "-alive_value <value>"

This means to eliminate duplicate elements and use the element where
<variable> has value <value> as the "canonical" element at that time
step.

Probably throw an error if more than 1 of the duplicate elements has
the value <value>...

Note that this issue can be avoided if user epu's individual files
first and then uses conjoin...
