Change `isxxx` and `hasxxx` to `is_xxx` and `has_xxx` (issue 1061).

The coding and design conventions (CDC) document was also updated.
Add entries to .gitignore file
Fix typos in docstrings
Change `isfront` and `isback` to `is_front` and `is_back`
Rename `center` to `centre` everywhere.

(I chose `centre` because there were more `centre` than `center`
around.)
Rename `GTestCase::passed` to `GTestCase::has_passed`.
Adapt `optimize.cpp` example to name change in `GOptimizerFunction`.
Use literalinclude in `opt.rst` docs.
Improve sphinx markup in user manual.
No `from gammalib import *` in Python examples
Remove extra non-implemented coordinate systems from GSkyProjection (fixes #1058).
Use galactic coordinates by default.
Remove explicit declaration for multi-argument constructors.
Merge branch 'issue_1062' into integration
Renamed m_passed to m_has_passed
Merge branch 'issue_1061' into integration
Update NEWS and ChangeLog after issue #1061.