Better grasping for xArm7

[s1lent4gnt]

What this does

This PR addresses the Graspin issue discussed in #83. The resolution includes the following changes:

• Updated the xarm7.xml, xarm7_nohand.xml, and hand.xml files by adding armature=0.1 to the xArm7 joints.
• Introduced two collision boxes as pads for each finger.

How it was tested

The changes were verified using the Mink Python library, with results documented in #83.

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[kevinzakka]

Thank you @s1lent4gnt! Can you add a little Changelog section in the README (see the unitree_g1 for an example) to describe succinctly the changes you made in this PR?

[s1lent4gnt]

Hey @kevinzakka, I’ve added references to both the Issue and this PR in the Changelog section. Does this look okay, or would you prefer not to include them?

[kevinzakka]

@yuvaltassa @saran-t ready to merge!

[yuvaltassa]

@yuvaltassa @saran-t ready to merge!

Great! Probably after the holidays 😄