merge with master

src/cell_group.hpp

src/fvm_cell.hpp

src/spike_source.hpp

src/stimulus.hpp

src/util/meta.hpp

src/util/singleton.hpp

tests/unit/test_fvm.cpp

tests/unit/test_probe.cpp

tests/unit/test_spikes.cpp

tests/validation/validate_ball_and_stick.cpp

tests/validation/validate_soma.cpp

tests/validation/validate_synapses.cpp