Add function to access popParameter String values by key in Parameters class

[Jannetty]

Right now filter is the only function you can use to access String values in the popParameters MiniBox in the Parameters class. Because filter is used to select all parameters with a certain prefix, it trims the term it searches from the key and returns a minibox. This makes it clunky for use when trying to access a single parameter value that only has one name; you are returned a minibox with one entry that has an empty string as the key and your desired value as the value.

I propose adding a function mimicking getInt and getDouble for strings. I will use this function to access the "class" population parameter from my cell constructors.

[Jannetty]

Update: with the changes I propose in #112, the filter function will no longer be able to return parameter values that have keys without tag separators. #115 addresses this issue.