Throw error for unknown variable name

[JamesPHoughton]

If you mistype a variable name, pysd should throw a fit, not just behave erratically.

[JamesPHoughton]

Related to: #58, both need to check that user entered strings asking for functions, setting values, etc are actually model elements.

[JamesPHoughton]

Also related to #115

[enekomartinmartinez]

I will try to include this as well in 3.8.0. Removing also case and whitespace sensitivity for original names.