Homogenize load_csv and load_npy

[wolfv]

Currently, load_csv takes a istream and load_npy takes a std::string for the filename. Note that the methods in xtensor-io also take a string filename.

We should homogenize the interfaces so that:

• load_* and dump_* takes a filename
• loads_* and dumps_* take a stream ? -- to be discussed
• and then also document how to create a stringstream for feeding strings as input

We should take this into considerat

[xiuliren]

I was confused by the interface, it takes some time to figure it out. It would be nice to be consistent and have documentation/examples.

[SylvainCorlay]

Thanks @jingpengw let us know if you feel like giving it a stab!

[liurui39660]

Yes, this is quite annoying when transferring from numpy to xtensor. Is there any plan for this issue?