Some numerical fixes

Type "float" is replaced with the PROPTYPE macros everywhere in the
code, and the macros is set to mean "double" in types.h.

Substraction-addition numerical error bug in TotalPropensity-level
sampler is fixed.

tests/testRelation.cpp added - was forgotten in its due time.

	modified:   population.cpp
	modified:   population.h
	modified:   randomGenerator.cpp
	modified:   randomGenerator.h
	modified:   reaction.cpp
	modified:   reaction.h
	modified:   relation.cpp
	modified:   relation.h
	new file:   tests/testRelation.cpp
	modified:   totalPopulation.cpp
	modified:   totalPopulation.h
	modified:   types.h