Describe the bug

Documentation issue

The equation (14) in equilibria.rst provides the Lao polynomial parameterisation for the plasma profiles.

The equation, however, disagrees with Lao et al. [1] and the docstring in bluemira/equilibria/profiles/laopoly. I believe that the equation should instead read:
$g\big(\overline{\psi},\boldsymbol{\alpha}\big) = \sum_{n=0}^{N}\alpha_{n+1}\overline{\psi}^{~n}-\overline{\psi}^{~N+1}\sum_{n=0}^{N} \alpha_{n+1}$.

That is, the first normalised $\psi$ should be to the power of $n$, not $n+1$.

However, rewriting it in keeping with Lao et al. may reduce confusion further:
$g\big(\overline{\psi},\boldsymbol{\alpha}\big) = \sum_{n=0}^{N}\alpha_{n}\overline{\psi}^{~n}-\overline{\psi}^{~N+1}\sum_{n=0}^{N} \alpha_{n}$.
Where $N$ is the number of coefficients -1: $N = |\alpha| -1$.

e.g. for $\alpha=[1, 2]$, $N=2-1=1$ and the equation becomes $g(x) = (1x^0 + 2x^1) - x^2(1+2)$

Implementation issue

The final term of bluemira/equilibria/profiles/laopoly raises the normalised equilibrium to N+2 (see the above definition) but I believe it should be to N+1.

Because N is defined as the number of constants minus one, this final term of the equation raises the normalised psi to the power of the number of alpha constants. However, the implementation does x ** (len(args) + 1) which is essentially raising it to $N+1 +1$.

[1] Lao, L. L., et al. "Reconstruction of current profile parameters and plasma shapes in tokamaks." Nuclear fusion 25.11 (1985): 1611.