Fix bug in TAxis::Copy

The TAxis::Copy function did not correctly handle the case where
not all bins had labels.
Implement TH1::Clone

The previous Clone method called for a histogram relied on both
the I/O system and on Cint to function. This lead to either
data race conditions or synchronization points when working with
different  histograms on different threads. Using TClass to
create an empty instance and then calling Copy plus a bit of
cleanup allows different histograms to be uncoupled.