TASK: Add assertions/sanity checks on input and output dimensions

[HenrikBengtsson]

Add stopifnot()-like assertions to validate consistency of input dimensions:

• [x] get.W()
• [x] get.Z()
• [x] initializeZt()
• [x] positiveFusedLasso()
• [x] getToyData()

Add stopifnot()-like assertions to validate consistency of output dimensions:

• [x] get.W()
• [x] get.Z()
• [x] initializeZt()
• [x] positiveFusedLasso()
• [x] getToyData()