remove need for SIRF_PATH and SIRF_INSTALL_PATH env variables

[KrisThielemans]

We still use the SIRF_PATH env variable explicitly in 2 places in the C++ tests where we should use examples_data_path() instead

SIRF/src/xSTIR/cSTIR/tests/test3.cpp

Lines 51 to 52 in 5ec1fac

	std::string SIRF_path = std::getenv("SIRF_PATH");
	if (SIRF_path.length() < 1) {

SIRF/src/xSTIR/cSTIR/tests/test5.cpp

Lines 50 to 54 in 5ec1fac

	std::string SIRF_path = std::getenv("SIRF_PATH");
	if (SIRF_path.length() < 1) {
	std::cout << "SIRF_PATH not defined, cannot find data" << std::endl;
	return 1;
	}

(It is also used in the obsolete matlab code, so let's ignore that).

In fact, we should not have to rely on the STIR_INSTALL_PATH env variable either. This should be resolved by making a .h.in file setting something like

const char * const SIRF_INSTALL_PATH = "@CMAKE_INSTALL_PREFIX@";
const char * const SIRF_INSTALLED_EXAMPLE_DATA_PATH = "@SIRF_EXAMPLE_DATA_PATH";

where the latter is set in CMake, rewriting the following slightly:

SIRF/CMakeLists.txt

Line 205 in 6fc60a4

set(SHARE_DIR ${CMAKE_INSTALL_PREFIX}/share/SIRF-${VERSION_MAJOR}.${VERSION_MINOR})

SIRF/CMakeLists.txt

Lines 228 to 230 in 6fc60a4

	install(DIRECTORY ${CMAKE_SOURCE_DIR}/examples
	COMPONENT DOC
	DESTINATION ${SHARE_DIR})

After that, examples_data_path() should be rewritten to only use the optional SIRF_DATA_PATH env variable and the new SIRF_INSTALLED_EXAMPLE_DATA_PATH.

Note that the current corresponding doc is not quite accurate. This paragraph should be removed I believe

SIRF/examples/README.md

Lines 41 to 43 in 5ec1fac

	Alternatively, `SIRF_PATH` should be set by the user to the full path to the root folder containing the SIRF source,
	and `SIRF_INSTALL_PATH` to the full path to the folder where the user installed SIRF binaries and other files (i.e.
	set by `CMAKE_INSTALL_PREFIX`).