DVA_VL: Calculate degrees of visual angle
In VinLaflamme/nlregVL: Non-linear fits for psychometric functions

Calculate degrees of visual angle

1 2	DVA_VL(stimX, stimY, dist, distUnit = "cm", scrX, scrY, scrUnit = "mm", resX, resY)

`stimX`	Stimulus width in pixels
`stimY`	Stimulus height in pixels
`dist`	Distance from screen to user
`distUnit`	Measurement unit for dist
`scrX`	screen width
`scrY`	Screen height
`scrUnit`	Measurement unit for scrX and scrY
`resX`	Screen resolution (horizontal)
`resY`	Screen resolution (vertical)