Resolve "Integrate Counter and MV register in the map"

Closes #12 (closed)

• Transform LWW map string/string to LWW map string/heterogeneous types (bool, int, double, string)
• Create a MV map supporting heterogeneous value types (bool, int, double, string)
• Create a generic map with heterogeneous type and heterogeneous merging strategies (lww, mv, counter)
• Add documentation and tests for these three maps
• Remove inconvenient kclass parameter in toJson fromJson for the LWWRegister