CRAN recomendations

[andyquinterom]

Please add \value to .Rd files regarding exported methods and explain
the functions results in the documentation. Please write about the
structure of the output (class) and also what the output means. (If a
function does not return a value, please document that too, e.g.
\value{No return value, called for side effects} or similar)
Missing Rd-tags:
print.tower.Rd: \value