More explicit error when TreeNode description is null
More explicit error message for non-unique leaves and missing branch len
Improve grammar for parsing trees

- use a simplified grammar
- number should not start by E/e (o.w. could be confused to node name)
- taxa name should not start by -/number (o.w. could be confused to len)

Fix #16
Add test for tree parser
Fix dependency of previous commits of today
Fix #17
Clean-up TestGenerateUnrooted

Note: the test might be at the boundary of failing, inspect and adjust
later if problem arises again
Improve doc on tree parser
Add auto test notifications