Adding an optional getname_as_string function

[isaacsas]

One issue with MTK symbol names is the need to use the unicode lower plus in hierarchical systems since a period can't be used, i.e. "act₊vol₁₊x".

Having a getname_as_string function would allow plot recipes, Latexify, and other formatters to have a systematic way to get a nicer representation of the variable/parameter name for display in output (i.e. SciMLBase could then use getname_as_string for plot recipes, and one could get a nicer act.vol₁.x type representation).