Row major format

[egpbos]

Data arrays in FFTW must be in row major format (http://www.fftw.org/fftw3_doc/Row_002dmajor-Format.html). Currently, xtensor-fftw defines output arrays as row major, but it does not check for row major-ness on the input arrays. Should this be done or do we leave it to the user?

[egpbos]

In the meantime, in some places, I have made the xarrays explicitly row-major. This should be done everywhere. Especially now that xtensor 0.14 has added full column-major support, we should make the basic functions explicitly row-major. Conversion from column- to row-major arrays should probably be xtensor functionality.

[egpbos]

It should be added to the documentation as well though.

[egpbos]

Added to documentation.