[on hold] Support for sub arrays of dynamic size (jagged arrays)

[bernhardmgruber]

This PR adds support for fields of dynamic array type. Specifically:

• allow dynamic field types in the record dimension
• add specializations to most of the core functions
• add llama::dynamic to signal a dynamic array member in a RecordCoord
• extend VirtualRecord to allow holding dynamic indices
• extend blobNrAndOffset to allow for additional dynamic indices
• add OffsetTable mapping
• add customization allowing to dump OffsetTable mappings
• add a few unit tests

[bernhardmgruber]

Given a small HEP style data structure:

    struct run {};
    struct luminosityBlock {};
    struct Electrons {};
    struct Muons {};
    struct Eta{};
    struct Mass{};
    struct Phi{};

    using Electron = llama::Record<
        llama::Field<Eta, float>,
        llama::Field<Mass, float>,
        llama::Field<Phi, float>
    >;
    using Muon = llama::Record<
        llama::Field<Eta, float>,
        llama::Field<Mass, float>,
        llama::Field<Phi, float>
    >;
    using Event = llama::Record<
        llama::Field<run, std::int32_t>,
        llama::Field<luminosityBlock, std::int32_t>,
        llama::Field<Electrons, Electron[]>,
        llama::Field<Muons, Muon[]>
    >;
....
    auto mapping = llama::mapping::OffsetTable<llama::ArrayDims<1>, Event>{
        llama::ArrayDims{30},
        llama::ArrayDims{50},
        llama::ArrayDims{40}};

Here is a dump of the mapping using AoS for the sub mappings:

Using SoA for the sub mappings:

Using Aos for the events and SoA for the electrons and muons: