Fix get_mean() for PDs, fix installation docs

[ilopezgp]

The current output of get_mean(ParameterDistribution) is an Array{FT, 2} that cannot be used as an input to the Sampler(). I did not see any instance of the code where this structure was useful, so I have changed the function to return an Array{FT, 1}.

Note that this change simplifies every single use of get_mean(ParameterDistribution) in the tests and examples.

Apart from this, I have corrected the installation guide.

[odunbar]

We store samples as oriented 2D arrays, viewed as lists of columns e.g if you have samples of data they are always stored as columns, I expect this was the mindset as to why the mean should be a column. However as the statistics mean function naturally reduces the dimension, it is definitely sensible to store it as a Vector{FT} and I believe we do also this for other "means" in the code too.

so LGTM

[ilopezgp]

bors r+

[bors]

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