support operators for umxRAM and plot

[tbates]

umxRAM and plot

• Automate?: Instead of supporting product = "xy", support productVars = c("x", "y") and auto create the xy & x,y -> xy paths with appropriate fixin's so user are not distracted by implementation.
• Add 'product' parameter to the created mxModel.
• include 'product' objects in addition to list of manifests and latents
• Figure out and implement requirements for fixing paths from manifest to operator result objects. (Seems to involve inputs to operators being forced to have mean = 0??)
• Check mean = 0 in vars used to created products?

plot

• Find operator means (currently in M)
• Decide on a graphic representation of the operator node (circle with name= operator?)
• Add to plot vertex and edge list

Example (plane-jane regression with interaction)

p1 = lm(mpg~wt*disp, data = mtcars); umxAPA(p1)
(Intercept) B = 44.08 [37.68, 50.48], t = 14.11, p < 0.001
wt B = -6.5 [-9.19, -3.81], t = -4.95, p < 0.001
disp B = -0.06 [-0.08, -0.03], t = -4.26, p < 0.001
wt:disp B = 0.01 [0.01, 0.02], t = 3.6, p = 0.001

productInputs = c("x", "y")
manifests = c(productInputs, "z")
m1 = umxRAM("testPoV", data = povData, productVars = productInputs,
    umxPath(c("x", "y", "xy"), to = "z"),
    umxPath(cov= c("x", "y")),
    umxPath(v.m. = manifests),

   # automatic for the user
   # 1. remove auto-named products from the latents list
   # 2. (add?) fixed@1 paths from productInputs to product 
    umxPath(c("x", "y"), to = "xy", fixedAt = 1),
   # 2 go back into model and fix means productInputs to zero (having checked they are that)
   #   model@matrices$means$free, productInputs], FALSE)
)