Avoid double cloning of histograms
First round of optimisations for the pch generation

a factor of 2 was gained in speed. Not clear how it got so slow
in the past. Probably because of the implementation of the features
of the DictSelection.
The timing for a debug build went from 460s to 255s.
Correctly do not assume order of iotype and ioname properties

this lead to not compilable dictionaries.
Cache access to hottest selrules properties
Avoid useless assignments, casts, std::arrays

further optimise dictionary generation runtime.
Mark the file entry invalid, until reread. Invalidate the SLocEntry cache and readd it on reread.
If the content cache of the SLocEntry is gone, the file was unloaded.
Simplify unloading of files, by delegating to cling.
Files that are .x-ed must be unloadable too. Simplify.
Adapt the user of invalidateCache to its new signature.
Note: this might be change of behavior, because the new implementation doesn't
erase the entry from the ADT.
Skip removing the system function _Unwind_Resume, needed by the JIT.
Adapt to the new (non-const) interface.
Another last bit of perf.
Use comparison not assignment.
Add FIXME.
Improve encapsulation.
Improve doc style.
Move the typedef definition before private section.
Shorten the unnecessary long interface name.
Style.