ign command version comparison needs natural sort order

[peci1]

Environment

• OS Version: Ubuntu 18.04
• Source or binary build? Binary 1.1.0

Description

• Expected behavior: ign sdf launches the latest installed sdformat command
• Actual behavior: It launches an older one.

Steps to reproduce

1. install libsdformat8-dev
2. install libsdformat10-dev
3. take a SDF 1.7 model and pass it to ign sdf --check.
4. The check fails saying that SDF 1.7 is too new for this tool to work with. That points at sdformat 8 being used.

I bet the problem lies in

https://github.com/ignitionrobotics/ign-tools/blob/d32853a9b4ebf21c3421f7e903e51a9c45e15849/src/ign.in#L145

which looks like C-order rather than natural order. But as my Ruby knowledge is limited, I don't know how to fix it correctly.

My system reports these version of sdformat:

8.9.1
10.3.0

[scpeters]

we can copy some of the ruby code from homebrew's version.rb file to handle version parsing and sorting

or we could try to add swig bindings for the ignition::math::SemanticVersion class

[chapulina]

add swig bindings for the ignition::math::SemanticVersion class

That's part of gazebosim/gz-math#101

Note that it would add a dependency to ign-math, and this may cause a circular dependency if we ever decide to add ign-tools to ign-math as part of gazebosim/gz-math#198.

[caguero]

See pull request #50 .