Changed evenlySpacedPhi() to default to linear rather than cubic interpolation.

When adjacent data points are very very close, cubic interpolation can
create enormous oscillations for small numerical (or even rounding /
floating point) errors. This happened occasionally for thin-walled
bubbles. These oscillations would manifest as big spikes in dphi/dr
which can then get fed into and break the path deformation routine.

CosmoTransitions v1 used linear interpolation in evenlySpacedPhi(). At
some point in development I must have not-so-cleverly thought that
cubic interpolation would be better.