Refactor Kernel::Interpolation to correctly handle multiple loading from string

[MichaelWedel]

When an instrument definition contains a FitParameter with a lookup-table, all values are inserted into the table twice (in some situations even 4 times).

I have traced this down to LoadInstrument::exec():

174      // populate parameter map of workspace 
175      m_workspace->populateInstrumentParameters();
176
177      // check if default parameter file is also present, unless loading from
178      if (!m_filename.empty())
179        runLoadParameterFile();

Both line 175 and 179 execute the >> operator for Interpolation (eventually), on the same instance, so the data stored in the IDF are added twice.

Adding a clearData() method to Interpolation and calling that inside the operator fixes the problem, but maybe it's possible to track down the real origin of this behavior.

If anyone knows details of this loading procedure, I would be happy about advice.

[MichaelWedel]

This issue was originally trac ticket 9682