Extend LEMS with *optional* max/min/step for Parameters/Constants/StateVariables

Motivation: to allow TVB/RateML (and other simulators) to specify recommended max/min for parameters in a valid LEMS file, e.g.
https://github.com/the-virtual-brain/tvb-root/blob/a2d64896bb8fa9ea7da3c10d57b177bcb0889101/scientific_library/tvb/rateML/XMLmodels/kuramoto.xml#L6

This would probably result in something like:

OLD: <Parameter name="global_speed" dimension='-1.0, 1.0'/>
NEW: <Parameter name="global_speed" min_recommended_value='-1.0' max_recommended_value='1.0'/>

also for Constants:

<Constant name="x0"  value="-1.6" description="..." min_recommended_value='-2.0' max_recommended_value='2.0' recommended_step='0.1'/>

and StateVariables:

<StateVariable name="y1" exposure="None" min_expected_value='-2.0' max_expected_value='2.0'/>

Note:

• These should be treated in implementations as optional metadata, and can be ignored by the simulators, etc. hence "recommended" and "expected".
• They will be particularly useful for generating GUIs etc. to give recommended values for sliders, setting bounds in plots etc.
• I will aim to add these to the Java impl to throw warnings if the bounds are exceeded.

FYI @DeLaVlag @maedoc