Added new member/ attribute 'chain_type' to chain

modules/mol/base/doc/editors.rst

modules/mol/base/doc/entity.rst

modules/mol/base/pymod/export_chain.cc

modules/mol/base/pymod/export_editors.cc

modules/mol/base/src/CMakeLists.txt

modules/mol/base/src/chain_base.cc

modules/mol/base/src/chain_base.hh

modules/mol/base/src/chain_type.hh

modules/mol/base/src/editor_base.cc

modules/mol/base/src/editor_base.hh

modules/mol/base/src/impl/chain_impl.cc

modules/mol/base/src/impl/chain_impl.hh

modules/mol/base/tests/test_chain.cc