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.

(cherry picked from commit cff60155a5aaa965169b97214ee4c75992db245f)
Fix TH1::Clone copy of fFunctions

(cherry picked from commit ba8a1e81e93286adb57ca49abf52bc8387dd96f8)

Conflicts:
	hist/hist/src/TH1.cxx
Add missing comment
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