python builtins: allow max, min

[scpeters]

Only a subset of python builtin functions are permitted in xacro ${} expressions for security reasons, for reasons listed here. I recently wanted to use min and max in a xacro file, so I wonder if they can be added to this list.

I looked at the list of python builtin functions that could be useful for math operations, and many of them have alternatives since we import math.*:

• abs: use math.fabs
• len: use .__len__() object method
• pow: use **
• sum: use math.fsum

The only other one I would consider adding is round. You can get partial functionality with math.floor and math.ceil, but not the same. Currently, though, I just need max and min. I would propose back porting this to kinetic if it is acceptable.

cc @sloretz @clalancette

[codebot]

This sounds reasonable. Let's create a melodic branch and start putting stuff there. I'd also like to finally get the unicode PR merged in for melodic.

[scpeters]

melodic sounds reasonable to me; though I'm not boss enough to create a melodic branch on this repo

[codebot]

I just created a melodic-devel branch.

[scpeters]

#179 made for melodic

[rhaschke]

Manually merged due to conflicts.