TargetHeadWellString doesn't solve in confined aquifer

[mbakker7]

TargetHeadWellString solves in a semi-confined aquifer, but not in a confined aquifer, throwing an slice indices must be integers or None or have an __index__ method error when solving.

Example to reproduce:

import timflow.steady as tfs
z = [1.0, 0, -20]
ml2 = tfs.ModelMaq(kaq=10, c=1000, z=z, topboundary="semi", hstar=0)
#ml2 = tfs.ModelMaq(kaq=10, z=z[1:])
ws = tfs.TargetHeadWellString(ml2, [(0, -10), (0, 10)], rw=0.1, hcp=-2, xcp=10, ycp=0)
ml2.solve()

Removing the # shows the error.

[mbakker7]

Fixed with #62