set clon and clat to 0 only for global files

if clon is None and clat is None

Closes #7 (closed)

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes

ping @Casthardi, this solves the wrong focus of the orthographic plots of regional files (see #7 (closed)).