enable multiple creation of heat exchanger

# Conflicts:
#	CHANGELOG.rst

…elop

…kshop

…up build system matrix

- fixing slack equation position
- adding a new relevant idx branch to identify circ pumps

…p_branch_model

…branch_model

# Conflicts:
#	pandapipes/component_models/abstract_models/branch_models.py
#	pandapipes/idx_branch.py
#	src/pandapipes/component_models/abstract_models/circulation_pump.py
#	src/pandapipes/component_models/circulation_pump_pressure_component.py
#	src/pandapipes/component_models/junction_component.py
#	src/pandapipes/idx_node.py
#	src/pandapipes/pf/build_system_matrix.py
#	src/pandapipes/toolbox.py

…_branch_model

# Conflicts:
#	src/pandapipes/pipeflow.py
#	src/pandapipes/test/api/test_components/test_circ_pump_mass.py

- required test adaption to ensure test passing

- no mass creation in circ pumps
- required test adaption to ensure test passing

…branch_model

# Conflicts:
#	src/pandapipes/idx_branch.py
#	src/pandapipes/pf/build_system_matrix.py
#	src/pandapipes/pf/derivative_calculation.py
#	src/pandapipes/pipeflow.py
#	src/pandapipes/test/api/test_components/test_circ_pump_mass.py
#	src/pandapipes/test/pipeflow_internals/test_pipeflow_analytic_comparison.py

… into feature/circ_pump_branch_model2

# Conflicts:
#	setup.py
#	src/pandapipes/component_models/component_toolbox.py
#	src/pandapipes/pf/build_system_matrix.py
#	src/pandapipes/pipeflow.py