IsDenseMatrix for transposed matrix-view

Summary

This MR adds operator[i][j] access to TransposedMatrixView that swaps the indices when accessing the wrapped matrix. Additionally it provides size information .N() and .M() again with swapped information from the underlying wrapped matrix. The element access is only implemented if the wrapped matrix is dense. The traits that classifies a dense matrix is better documented such that also the TransposedMatrixView can be seen as a dense matrix if the underlying wrapped matrix is dense.