Include parameter for specifying integration order in integration workflow

[allyhawkins]

Based on #274 (review), we should add a parameter to the integration workflow to dictate the order to use for integrating libraries: based on the input order, random, or auto using auto.merge. I believe this would only be used for fastMNN integration and would not apply to harmony.

[jashapiro]

We might want to test in harmony to see if the order of input matters. An "auto" option is not available there, but if we are going to include "random" we could pretty easily do that for harmony too. The only caveat there (which maybe isn't small) is that while fastMNN will preserve input order in output for samples no matter what is given to merge.order and/or auto.merge, actually reordering the samples will change the order for the output and would therefore require resorting after integration.