In the case of LoadEventPreNexus is is "EventFilename". It seems that for
everything else is is "Filename" because the variable that holds this is
empty by the time the algorithm comes to run (though the loader is
selected correctly).
I've factored out the method that sets the variable and called it again
from within a method that exec() calls.

- It appears I only call release() if the elementTag == 1. I should call it otherwise to. This will prevent the memory leak.

The base component (m_base) has to be of type Instrument; therefore use
a static_cast in place of a dynamic one.

This fixes the issue of always getting zero back for a parameterized
instrument.

And a passing one in InstrumentTest (for a non-parameterized
instrument) as that's missing.

It seemed rather heavyweight to launch and plough through most of
LoadIDFFromNexus & ExperimentInfo::loadNexus when the thing we want
isn't there anyway.

The strings will be empty on construction anyway.

Previously, this failure would just log and the calling algorithm
thought it had succeeded when it hadn't.

Refs #7557

Refs #7557