Error thrown when calling `rand(d::GaussianLaguerre, dims::Dim2)` method

While writing docstrings for `rand` methods I found this:
```
julia> rand(GaussianLaguerre(2, 1.0), (3, 3))
ERROR: UndefVarError: `a` not defined in `RandomMatrices`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
[1] rand(d::GaussianLaguerre, dims::Tuple{Int64, Int64})
@ RandomMatrices ~/Documents/Julia Packages/JuliaMatrices/RandomMatrices.jl/src/GaussianEnsembles.jl:180
[2] top-level scope
@ REPL[54]:1
```
Originates from not accessing `a` from the object `d`. Tests should be added for this method too as it looks a little suspicious as is:
https://github.com/JuliaMath/RandomMatrices.jl/blob/c76044d43e718900e72fe71266955c65a47258b0/src/GaussianEnsembles.jl#L149-L173

Start in src/GaussianEnsembles.jl, especially the rand method around lines 149-180, and reproduce the GaussianLaguerre call shown in the issue. Verify the parameter access and add tests for the rand method, then run the relevant package tests to confirm the call succeeds.

索引モデルが issue の本文から書いたものです。