Add explicit to single argument constructors

[naterpotatoers]

As a best practice, all one-parameter constructors should be stated with the explicit keyword to vaoid behind the scenes type conversions e.g.

Object obj;
obj = 37;

This code snippet would construct an Object call obj by using the = statement which is probably not what we would want with our classes

[naterpotatoers]

Drive rover system needs explicit keyword added