NPBBBdesigns 1.0.0

• First release.
• Six construction methods for nested partially balanced bipartite block (NPBBB) designs: construct_method1 to construct_method6.
• Exact A-efficiency evaluation for the block and sub-block classifications via info_matrix, a_value, a_value_optimal and npbbb_efficiency, benchmarked against the A-optimal completely symmetric reference design.
• print methods for the "npbbb" and "npbbb_efficiency" classes, including a full physical block / sub-block layout.
• n_units helper for the total number of experimental units.