use STIR's sum(), find_max(), operator+= etc

[KrisThielemans]

We currently implement our own max() functions etc.

SIRF/src/xSTIR/cSTIR/stir_data_containers.cpp

Lines 512 to 515 in 63be16b

	STIRImageData::max(void* ptr) const
	{
	#if defined(_MSC_VER) && _MSC_VER < 1900
	Image3DF::const_full_iterator iter;

However, STIR has equivalents (e.g. find_max() and will soon optimise these. It would therefore be better to call them instead.

This could maybe be done by moving some of the generic implementations to DataContainer, and overriding for the STIR data containers, although that might be more complicated.

[evgueni-ovtchinnikov]

Do we really need operator+= etc. in C++ (already have in Python)? If not, then we are done.