feat(import-export): EPSG not mandatory and fix encoding issue

[cbourget]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines:https://github.com/infra-geo-ouverte/igo2/blob/master/.github/CONTRIBUTING.md#git-commit-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What is the current behavior? (You can also link to an open issue here)

• "EPSG" is mandatory in the EPSG code
• Encoding issue may happen when exporting files with ogre
• GPX files are exported with ogre and it's not so good at it

What is the new behavior?

• "EPSG" is not mandatory in the EPSG code
• No more encoding issue when exporting files with ogre
• GPX files are exported using OL. Some files that ogre wouldn't process now work perfectly fine

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications:

Other information: