Simplify HierarchicalTimer implementation

[jsiirola]

This issue documents possible TODOs for the HierarchicalTimer implementation that could be "nice to haves", but shouldn't hold up PR #96:

• Remove redundancy in the string representation. Significant logic (and embedded constants) are duplicated between HierarchicalTimer.__str__() and _HierarchicalHelper.to_str(). It is possibel that we might be able to accomplish this by refactoring the logic to only have a single class.
• Switch __str__ indententation to be a consistent (4?) spaces. This will allow for longer identifiers without causing the table to become too wide to easily display.
• Add a check to prevent users from including separators ('.') in identifier names