Export baseInstrument to python

The baseInstrument has the information of the positions of the detectors
before loading any calibration values. Using the information from this
object it will not require disabling loading the calibration values any more.

re #7491
Use the detectors from baseInstrument

It nows use the position from the baseInstrument which is free from any
previous calibration being applied to the detectors.

The only issue that it introduces is that you can not calibrate instruments
that was moved or rotated before the acquisition of the data. Because,
the baseInstrument does not consider this. Besides, any previous movement
for the detector will be reset when applying the output of the calibration.

But, we consider these limitations better than having to switch off loading
the calibration values through comment the region det-pos-source.

re #7491
Define the fitPar from the baseInstrument

It is wrong to take the length from the calibrated tube and applying to
the baseInstrument.

re #7491
Correct the Merlin Example

There were lacking tubes inside the door3.

re #7491
Merge remote-tracking branch 'origin/feature/7491_reset_calibration'