remove dependency on Rcpp; ignore src/*.so

- we don’t need to depend on Rcpp for just one function, wherein we
don’t even use any of the Rcpp classes.

- also add ‘flowWorkspaceData’ to the Suggests: section of DESCRIPTION,
for local vignette building
Add 'side' argument to 'cytokine' #63

- by default, we take side='right', which uses behavior as before
(identifies the right tail); however, we allow the use of side='left',
which attempts to identify the left tail.

- essentially implemented by flipping the operations done for
identifying the right tail
some R CMD check silencing

- roxygen complains unless "setClass("fcPolygonGate", ...)" is moved up
- force the vignette to explicitly check for 'flowWorkspaceData'
- documentation still needs some tuning