Sort variable indices from MCMCChains ourselves

[sethaxen]

Resolves issue where depending on how an MCMCChains.Chains was created, slicing variables from the underlying AxisArray may or may not return the subarray with the variables in the same order we requested. To avoid the issue, now we don't slice at all. Fixes #138.

[sethaxen]

@AnselmJeong can you verify that this PR fixes the issue you raised?

[AnselmJeong]

I cannot help but be amazed by your enthusiasm for your project....

I check the code and verified that the two results summarystats(chain) and summarystats(from_mcmcchain(chan)) coincide with each other. I compared the obtained parameter values and the original data, and concluded that the newest code report the parameter in correct order.

Thanks again....

[sethaxen]

I cannot help but be amazed by your enthusiasm for your project....

Thanks for the kind words! It helps to hear from users, and thanks so much for raising this issue so we could fix it.

I check the code and verified that the two results summarystats(chain) and summarystats(from_mcmcchain(chan)) coincide with each other. I compared the obtained parameter values and the original data, and concluded that the newest code report the parameter in correct order.

Great! Thanks for confirming.