Change Modifications to be its own class

[GrmanRodriguez]

This PR changes the way we write modifications in the urdf-modifiers library.

Modification is now a class part of urdfModifiers.core with methods for adding the different modifications.

A constructor for creating this class from a section of an ini file has also been added.

Below some examples of the changes:

Previous way of creating modifications:

modifications= {}
modifications[utils.geometry.Modification.DIMENSION] = [0.2, utils.geometry.Modification.ABSOLUTE] 
modifications[utils.geometry.Modification.DENSITY] = [2.0, utils.geometry.Modification.MULTIPLIER]

New method:

modifications = Modification()
modifications.add_dimension(0.2, absolute=True)
modifications.add_density(2.0, absolute=False)

As you can see it is now clearer and the code is cleaner.

cc @CarlottaSartore

[traversaro]

fyi @fabiodinatale

[vvasco]

LGTM!

[traversaro]

@GrmanRodriguez @vvasco can we tag the version of the library before this PR as v0.0.1 or something similar? we have some scripts that use the old modifications API, and while we could refer to specific checkout, it may be easier to refer to a given tag. No release is necessary, just a small tag is ok.

[vvasco]

good idea @traversaro thanks

[vvasco]

I'm creating a tag v0.0.1

[GrmanRodriguez]

In that case I'd say first accept #12 since it's non-breaking and adding it to the tag

[vvasco]

In that case I'd say first accept #12 since it's non-breaking and adding it to the tag

I agree. Then after that PR is accepted and merged, we can proceed with the tag.

[AlexAntn]

LGTM!

PR #12 has been merged, so we can create the tag and then merge this PR too.

Great work @GrmanRodriguez !