add `enumerate`

Given an 'iterable', return a new iterable which iterates over a pair of index and elements of the original 'iterable'.

For example:

   for ( auto [i,s] : LHCb::range::enumerate( std::array{0,1,4,9,16,25} ) ) {
       assert( i*i == s );
   }

The index starts, by default, at 0, and has a type of size_t but this can be customised as well:

   for ( auto [i,s] : LHCb::range::enumerate( std::array{4u,9u,16u,25u,36u}, 2u ) ) {
       assert( i*i == s );
   }

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assigned to @gunther

/ci-test --merge --platforms=x86_64-centos7-gcc9-opt,x86_64-centos7-clang8-opt

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• [2020-09-19 16:12] Validation started with lhcb-master-mr#1343
• [2020-09-21 00:15] Validation started with lhcb-head#2717

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assigned to @graven and unassigned @gunther

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• b1a04f54...f518e1c7 - 12 commits from branch master
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assigned to @dovombru and unassigned @gunther