Fix a unit and update a term to be vectorized in ArrayEquations.HeatTransfer

[adrian-nilsson-fcc]

Fixes two bugs in the HeatTransfer examples.

Wrong unit for ConvectionCoefficient

Fixes #454

Missing vectorization

The first term of the derivative that uses vector notation has two problems:

1. it refers to the variable i, which is never declared in this example
2. it is written to result in a scalar rather than a vector

The solution presented in this PR is straightforward: Replace

 -h*A_s*(T[i]-Tamb)

with

-h*A_s*(T[2:n-1]-fill(Tamb, n-2))