v0.4-0

The main reason for the version release was to fix issue #4, and I decide to update data sets as new features in the process.

• updated all data sets include latest release dates, to May of 2021.

• SP500 data series includes more variables from Robert Schiller's data set for U.S. Stock Markets 1871-2021

• updated documentation file for data series GDPC1, where the BEA changed a URL for its source citation.

• Fixed issue #4, added rmarkdown to suggests as it is no longer a dependency for knitr. The knitr author decided the package should no longer depend on rmarkdown, even though knitr clearly does depend on rmarkdown and will not function without it. This caused the vignette build to fail for this, and at least 388 other CRAN packages according to an email from CRAN. It appears a trend has been developing among devs working for certain companies and developer circles to move what are truly dependent packages to merely "suggests", as a way to circumvent issues associated with increasingly growing dependency trees. This shifts the developers burden to users to figure out, as was the case here. While this makes sense for external packages that add select features and some additional functionality, I would opine it seems less apropriate for those packages that are critical to the operation of the core functions, like building vignettes.