Simplify the Xtext grammar for the definiton of ROS parameters

[ipa-nhg]

the ROS model grammar for the definition of parameters is with the current version the following:

Parameter { name 'myIntParam' type Integer { default 5 } }}}

It can be simplified to:

Parameter { name 'myIntParam' type Integer default 5 }}}

[ipa-nhg]

done in 563db69