Increase transparency when setting global input / parameters

[edwinnglabs]

Right now, we allow insertion of an input to the probabilistic programming dict happen anywhere within the forecaster workflow. However, developer who wants to extend a new model has no visibility what are the full list of default inputs will be inserted. Same story applied to sampling parameters. I wonder if there is any way to maintain a central all-in-one place to operate all those insertion logics. Meanwhile, we can also maintain a global constants to inform this if necessary.