Regression tests for correctly loading old intrinsic models

The testfile for loading in old files tests/regression_test/data/01_old_intrinsic_extModel.pet has already the new format

<INTRINSIC_PARAMETERS CALIB_FILES="" ENABLED="0" EXT_MODEL_ENABLED="1" FIX_CENTER="0" IMMUTABLE="0" QUAD_ASPECT_RATIO="0" TANG_DIST="1">
    <OLD_MODEL CX="551.5" CY="383.5" FX="881" FY="881" K4="0" K5="0" K6="0" R2="0" R4="0" R6="0" ReprError="nan" S1="0" S2="0" S3="0" S4="0" TAUX="0" TAUY="0" TX="0" TY="0"/>
    <EXT_MODEL CX="947.47000000000003" CY="698.44000000000005" FX="857.12" FY="857.50999999999999" K4="1.7399009" K5="0.39905" K6="-0.013337" R2="1.35895" R4="0.123464" R6="-0.0091120005" ReprError="0.52066356" S1="0.001026" S2="-6.6000001e-05" S3="-0.00019000001" S4="-9.7999997e-05" TAUX="-2.9999999e-05" TAUY="-5.0999999e-05" TX="0.000184" TY="-0.00038799999"/>
</INTRINSIC_PARAMETERS>

But should have the old format with no child nodes (example from another file!)

<INTRINSIC_PARAMETERS CALIB_FILES="" CX="952.03999999999996" CY="698.72000000000003" ENABLED="0" EXT_MODEL_ENABLED="0" FIX_CENTER="0" FX="842.57000000000005" FY="842.69000000000005" IMMUTABLE="0" K4="0" K5="0" K6="0" QUAD_ASPECT_RATIO="0" R2="-0.27662599999999998" R4="0.093287999999999996" R6="-0.01423" ReprError="1.1930544727952612" S1="0" S2="0" S3="0" S4="0" TANG_DIST="1" TAUX="0" TAUY="0" TX="-0.00024699999999999999" TY="5.5000000000000002e-05"/>