BUG: Swap byteorder in constructor of Block Manager (#4737)
- [x] closes #4737
- [x] tests added / passed (at least the test_internals.py)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This is one possible approach to directly change the dtypes to the native byteorder in the constructor of the BlockManager. This code does it automatically, without the user deciding a swap, so I am not sure if this is a wise design decision.
Another concern is the speed of conversion into a pandas object. I didn't quite grasp the asv stuff, so maybe someone else can check for me (sorry!).
I will add something to whatsnew, after you have confirmed, that this method of doing it is okay.

Please tell me if I forgot to do something.


