SPECT projection DICOM: support multiple heads

[KrisThielemans]

Currently we're getting

WARNING: num_projections (60) * num_energy_windows (2) != num_frames (240)

The header written is essentially only for the first head, i.e.

!number of projections := 60
!extent of rotation := 180
!process status := acquired
!SPECT STUDY (acquired data) :=
!direction of rotation := CCW
start angle := 180.041
orbit := non-circular
radii := {60 numbers here}

At present, we have to manually adjust the header

• double number of projections and extent of rotation
• append radii from relevant dicom field

Resolving this is relatively complicated, also because our Interfile support is incomplete #1102

[KrisThielemans]

DICOM info is at https://dicom.nema.org/medical/dicom/current/output/chtml/part03/sect_A.5.4.html