Refactor abstract containers away from concrete containers

src/MRIBuilder.jl

src/all_building_blocks/building_blocks.jl

src/all_building_blocks/wait_blocks.jl

src/all_sequences/all_sequences.jl

src/all_sequences/sequences.jl

src/containers/abstract.jl

src/containers/alternatives.jl

src/containers/building_blocks.jl

src/containers/containers.jl

src/containers/sequences.jl

src/parts/epi_readouts.jl

src/parts/helper_functions.jl

src/parts/parts.jl

src/parts/slice_select_rephases.jl

src/parts/spoilt_slice_selects.jl

src/parts/trapezoids.jl

src/pathways.jl