Add Kuka Kr 6 R1820

[simonschmeisser]

by @timobkt

[cjue]

• match filenames to other models in this package
• updated URDF
• use xacro instead of xacro.py
• added opw files for kr6r1820 and kr6r700six (the r700sixx config already existed with a different file name)

[cjue]

Note: the previously added opw config for the kr6r700sixx (and the KR5) does not follow the naming convention used elsewhere:

$ find . -iname opw_*.yaml
./kuka_kr6_support/config/opw_parameters_kr6r900_2.yaml
./kuka_kr6_support/config/opw_parameters_kr6r1820.yaml
./kuka_kr6_support/config/opw_kinematics_kr6r700sixx.yaml
./kuka_kr5_support/config/opw_kinematics_kr5.yaml
./kuka_kr150_support/config/opw_parameters_kr150r3100_2.yaml
./kuka_kr210_support/meshes/kr210_r3100-2/orig/opw_parameters.yaml
./kuka_kr10_support/config/opw_parameters_kr10r1420.yaml

Should we change the name of the file, or would that cause too much breakage for users of the current file?