ENH: native sparse conversion from csc/csr scipy sparse matrix to DataFrame

[jreback]

http://stackoverflow.com/questions/17818783/populate-a-pandas-sparsedataframe-from-a-scipy-sparse-matrix

[kernc]

• https://stackoverflow.com/questions/31084942/pandas-sparse-dataframe-to-sparse-matrix-without-generating-a-dense-matrix-in-m
• https://stackoverflow.com/questions/33457626/huge-sparse-dataframe-to-scipy-sparse-matrix-without-dense-transform
• https://stackoverflow.com/questions/41152264/convert-pandas-sparsedataframe-to-scipy-sparse-csc-matrix
• https://stackoverflow.com/questions/37084032/how-do-i-create-a-scipy-sparse-matrix-from-a-pandas-dataframe
  ...

[jreback]

@kernc there might be some other sparse issues which are covered / dupes by this. If you find any pls ping on them.

[kernc]

Surprisingly, no duplicates.