Set mesh.args to NULL and let the user choose some values

[simon-smart88]

Look at the source for prepare_data to see how they are calculated

[simon-smart88]

@timcdlucas I've implemented this, but this is all that was in build_mesh():

  maxedge <- hypotenuse/10
  pars <- list(convex = -0.01, concave = -0.5, resolution = 300, 
    max.edge = c(maxedge, maxedge * 2), cut = 0.1, offset = c(hypotenuse/10, 
      hypotenuse/10))
  pars[names(mesh.args)] <- mesh.args

So offset has two values, but they are both set to the same value which is the same as the lower bound of max.edge - is that correct?

I also needed to change the first line to maxedge <- as.numeric(hypotenuse/10) because it was a named number before so we may want to check that that would be caught by the tests in {disaggregation}

[simon-smart88]

This was dealt with in #21