Saving pixel match info in the electron for offline ID purposes

This pull request adds the pixel seed into to the GsfElectron object to enable better electron - photon disambiguation and miss matched tracks at the identification level. This change has been agreed by E/gamma (Lindsey, Matteo,Sam) and it would be nice if we could include it into 73X
Adding a class version detail:
```
<class name="reco::GsfElectron::PixelMatchVariables" ClassVersion="1">
    <version ClassVersion="1" checksum="515498629"/>
</class>
```

causes an error message at build time:

```
>> Checking EDM Class Version for src/DataFormats/EgammaCandidates/src/classes_def.xml in libDataFormatsEgammaCandidatesCapabilities.so
Automatic building of dictionaries now off
error: for class 'reco::GsfElectron::PixelMatchVariables' ROOT says the ClassVersion is -1 but classes_def.xml says it is 1. Are you sure everything compiled correctly?
gmake: *** [tmp/slc6_amd64_gcc491/src/DataFormats/EgammaCandidates/src/DataFormatsEgammaCandidates/libDataFormatsEgammaCandidatesCapabilities.so] Error 1
gmake: *** [There are compilation/build errors. Please see the detail log above.] Error 2
```

Is there a solution for this?