Fix GetSpike Tests

[sophiafarrell]

TravisCI was having a hard time occasionally with the GetSpike test. Probably a NEST issue, or we are feeding incorrect parameters in. Here's where it's defined:

nestpy/src/nestpy/NEST.cpp

Lines 1518 to 1520 in f68b74f

	vector<double> NESTcalc::GetSpike(int Nph, double dx, double dy, double dz,
	double driftSpeed, double dS_mid,
	const vector<double>& oldScint) {

And here is our test for it:

nestpy/tests/core_nest_tests.py

Lines 112 to 114 in f68b74f

	# def test_nestcalc_get_spike(self):
	# # This is stalling some builds. Need to improe the test.
	# self.nestcalc.GetSpike(10, 10., 20., 30., 10., 10., [0, 1, 2])

I have found testing this locally also can replicate the issue; code just freezes up. It's more important to get nestpy patch out than get this particular rarely-used function working but we should definitely fix before next patch.

[sophiafarrell]

@tunnell FYI. No TODO yet, but just aware of this tiny enhancement for our tests we'll need to fix.