Michael Mayer / gammalib

(I chose `centre` because there were more `centre` than `center`
around.)

The coding and design conventions (CDC) document was also updated.

For some reason most of the RST files use MAC `^M` instead
of UNIX `\n` newline characters.

This is problematic because e.g. git diff doesn't work with
MAC newlines by default and considers such files as one line,
giving meaningless results:
https://cta-redmine.irap.omp.eu/issues/558#note-10

I ran the mac2unix tool to convert MAC to UNIX newlines
as suggested here:
http://stackoverflow.com/questions/64749/m-character-at-end-of-lines
http://stackoverflow.com/questions/1889559/git-diff-to-ignore-m

Besides these RST files the GammaLib repo seems to be clean,
i.e. no other files changed when I ran mac2unix on them.

- Add more information
- State that GammaLib uses C++98, not C++11
- Change statement on supported Python versions
  (was 2.4 upwards, now 2.6 upwards)

Conflicts:
	doc/source/user_manual/modules/model.rst

The abstract pointing class has been removed as so far no abstract pointing interface is needed.