Default coefficient of ImplicitSourceTerm is 0

[guyer]

>>> import fipy as fp

>>> mesh = fp.Grid1D(nx=5)
>>> phi = fp.CellVariable(mesh=mesh)

>>> eq1 = fp.ImplicitSourceTerm(var=phi) == 3.
>>> phi.value = mesh.x
>>> eq1.solve()
python:1: StagnatedSolverWarning: The solver stagnated. Iterations: 1. Relative error: nan
  #!/bin/bash python.app

>>> print phi
[ nan  nan  nan  nan  nan]

>>> eq2 = fp.ImplicitSourceTerm(var=phi) == 3.
>>> phi.value = mesh.x
>>> eq2.solve()
>>> print phi
[ 3.  3.  3.  3.  3.]

Principle of least surprise says default coeff should be 1. Zero is useless.

[guyer]

While strictly not backward compatible, changing default coeff from 0 to 1 will not break anything that is not already broken.