Multiple projectors with SPECTUBMatrix cause double free error

[samdporter]

I think there's an error in freeing memory in the destructor for the ProjectorByBinPairUsingProjMatrixByBin class when used with the SPECTUB class

I'll include some minimal examples as well as valgrind output below.

Created a new issue here as opposed to SIRF issue #1101 as it's definitely a STIR issue.

I've tried recompiling SIRF/STIR without OpenMP as suggested in the issue above, but it hasn't made any difference.

[KrisThielemans]

Probably best to wait till #1169 is merged. That will change things.

[samdporter]

This repo now contains some minimal SIRF & STIR scripts that throw the error
double free or corruption (!prev) Aborted (core dumped)
The errors occur on exiting the script after setting up two acquisition models with SPECTUB matrices

The repo also contains some valgrind output files the I'll copy here:
for SIRF
for STIR

[samdporter]

Appears fixed by #1169

see updated repo