HistogramSpec: add missing L types

[ddobrigk]

This PR adds the missing TH1L, TH2L and TH3L types to the framework. Note that THnL sparses - the more costly options, incidentally - are already defined. The 64-bit longs are especially interesting when dealing with fast simulation, a context in which a massive amount of tracks will for sure be handled.

I'm actually having some trouble compiling this as the compiler says THxLs are not defined, which is puzzling: they are defined together in TH1.h in ROOT. Maybe I am missing something else? @aalkin when you have a moment, can you help me out? (but please do not let me disturb your weekend - only whenever you manage). Many thanks!

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[github-actions]

This PR did not have any update in the last 30 days. Is it still needed? Unless further action in will be closed in 5 days.

[github-actions]

REQUEST FOR PRODUCTION RELEASES:
To request your PR to be included in production software, please add the corresponding labels called "async-" to your PR. Add the labels directly (if you have the permissions) or add a comment of the form (note that labels are separated by a ",")

+async-label <label1>, <label2>, !<label3> ...

This will add <label1> and <label2> and removes <label3>.

The following labels are available
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[aalkin]

@ddobrigk could you also mark it as not draft, so the tests run?

[mhemmer-cern]

Any news on this one?

[ddobrigk]

Recreated (with a rebased branch) here: #14333 - let's see if we can manage to follow this up this time :-)