Occasional runtime warning

[j-c-cook]

I occasionally get the following run time warning.

H:\Masters\ghedt\ghedt\PLAT\pygfunction\pipes.py:736: RuntimeWarning: invalid value encountered in subtract
  if (np.all(np.abs(m_flow_borehole*cp_f - stored_m_flow_cp) < np.abs(stored_m_flow_cp)*tol)
H:\Masters\ghedt\ghedt\PLAT\pygfunction\pipes.py:736: RuntimeWarning: invalid value encountered in multiply
  if (np.all(np.abs(m_flow_borehole*cp_f - stored_m_flow_cp) < np.abs(stored_m_flow_cp)*tol)

[MassimoCimmino]

This may be related to the use of numpy.empty to initialize arrays, for example here:

pygfunction/pygfunction/pipes.py

Lines 616 to 623 in 73cb929

	def _initialize_stored_coefficients(self):
	nMethods = 8 # Number of class methods
	self._stored_coefficients = [() for i in range(nMethods)]
	self._stored_m_flow_cp = [np.empty(self.nInlets)
	for i in range(nMethods)]
	self._stored_nSegments = [np.nan for i in range(nMethods)]
	self._m_flow_cp_model_variables = np.empty(self.nInlets)
	self._nSegments_model_variables = np.nan

The proper way is probably to use numpy.zeros or numpy.nan.

[MassimoCimmino]

@j-c-cook Please let me know if the changes on the development branch issue155[...] fixes the issue.

[j-c-cook]

The error message shown in the original post is intermittent. I'm not sure what causes it, but it occasionally happens. I don't think I've seen it since I posted this 7 days ago. Given that I don't know how to reproduce the runtime warning, I cannot verify that your fixes have resolved the issue. My proposal is that we consider it "good enough", especially given that the runtime warnings never had an effect on results.