fix init of MeasurementProviderT to avoid segfault if UseBfield=true

As far as I understand it, MeasurementProviderT was calling initialize incorrectly, thus skipping MeasurementProviderProjector::initialize() which left the m_pIMF pointer to the MagneticFieldSvc uninitialized.
When using the option UseBfield the function internal_project would use this pointer and thus segfault 💣

new test for this in Moore!836 (merged)