mapLegend: Add a color scale legend to a heat map
In rBatt/rbLib: Library of RB's helper functions

Adds a bar that associates numeric values with colors. Useful for ay plot that uses a linear color gradient to indicate a numeric value, and for which there are too many colors/ values to specify each individually.

1 2	mapLegend(x = 0.9, y = 0.2, w = 0.05, h = 0.25, zlim, cols, horiz = FALSE, axSide, adj, offset, lab.cex = 1, lab.sig = 2)

`x, y`	the locations of the center of the legend, as a proportion of plot area (between 0 and 1)
`w, h`	the width and height of the legend, as a proportion of plot area
`zlim`	lower and upper limit of values indicated by colors
`cols`	colors used in the original plot; e.g., created by `zCol`. Note that it is assumed that color and the value change linearly
`horiz`	Logical, if FALSE (default), the colors in the legend change long the vertical axis
`axSide`	the side of the color bar on which the axis (line, ticks, labels) should be placed; if missing, reasonable default is chosen
`adj`	length 1 or 2 numeric vector to be passed to `text` that will affect the position of the tick marks; if missing, reasonable default is chosen
`offset`	length 1 numeric vector to be passed to `text` that will affect the position of the axis labels; if missing, reasonable default is chosen
`lab.cex`	numeric value to be passed to `cex` argument of `text` that will affect the size of the axis labels
`lab.sig`	integer, the number of significant digits to which the axis labels should be rounded; see `signif`

data(volcano)
v <- volcano
v[v<=quantile(v,0.2)] <- NA # small values to NA (will plot white)
v_cols <- zCol(256, 1:256)
image(v, col=v_cols)
zl <- range(v, na.rm=TRUE)
mapLegend(x=0.9, y=0.85, zlim=zl, cols=v_cols)
mapLegend(x=0.975, y=0.075, zlim=zl, cols=v_cols, w=0.025, h=0.125)
mapLegend(x=0.73, y=0.975, zlim=zl, cols=v_cols, w=0.125, h=0.025, horiz=TRUE)
mapLegend(x=0.93, y=0.25, zlim=zl, cols=v_cols, w=0.1, h=0.025, horiz=TRUE)