Foundation: doxygen typo fixes
TTValue: cleanup of some old cruft and adding a few comments.
TTValue: after a number of iterations of the code, we were left with a mixture of inconsistent at() and [] operator usage.  In most cases we are already bounds-checked or bounds-safe, so there is no need for the extra overhead incurred by calling at() instead of using the [] operator.
TTValue: adding comment about the reserve() call in the empty constructor.
TTValue: can now contain error codes -- this means we can return a value from a function and still return an error code if it fails instead of forcing us to use potentially clumsy function arguments for our return values.
TTValue: some optimizations for float64 conversion