Make sage -notebook=ipython land by default in pwd

[slabbe]

Since #16996 the default directory of sage -notebook=ipython is DOT_SAGE/notebooks_ipython/ following the old notebook. In the old Sage notebook, each worksheet was a directory and even each single cell was stored in its own directory. Therefore, there was no notion of "present working directory" at all. With the ipython notebook, each worksheet lives in one present working directory chosen by the user. That is one of the main point of the ipython notebook philosophy and we should not change that in Sage. See the post of Fernando Perez about difference between the ipython notebook and the Sage notebook:

"This is a key difference of our approach and the Sage notebook, so it' worth clarifying what I mean, the key point being the execution model and its relation to the filesystem. The Sage notebook took the route of using the filesystem for notebook operations, so you can't meaningfully use 'ls' in it or move around the filesystem yourself with 'cd', because Sage will always execute your code in hidden directories with each cell actually being a separate subdirectory. This is a perfectly valid approach and has a number of very good consequences for the Sage notebook, but it is also very different from the IPython model where we always keep the user very close to the filesystem and OS. For us, it's really important that you can access local scripts, use %run, see arbitrary files conveniently, etc., as these are routine needs in data analysis and numerical simulation." (Fernando Perez, ​The IPython notebook: a historical retrospective, January 2012)

Let's change the default directory so that it is equal to the present working directory (pwd) as it is the nice way of using ipython.

​See the related discussion and vote on sage-devel.

[slabbe]

My branch based on top of #16996 have conflicts. I will compile 6.4.beta6 during the night and fix the conflict tomorrow.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​e8f739c

Trac #17203: make pwd the default directory for the ipython notebook