For viewers of Showtime’s series “Billions,” the character Wendy Rhoades, an in-house hedge fund psychiatrist played by Maggie Siff, may seem to be a dramatic invention. After all, do Wall Street’s power players really need their egos boosted? It turns out that there is indeed a niche industry for Wall Street psychiatrists and performance coaches, intended to get traders and investors back on their game, to avoid the loss of potentially millions of dollars of other people’s money.