Add convert command

[sudo-nano]

Add a convert command that will convert measurements between units. For user friendliness, should support single argument notation like "1mL" and 2 argument notation such as "1 mL".

[sudo-nano]

The command's syntax has been added to the parser, but actually implementing its function requires implementation of the Mass class (analogous to Volume class, wraps value and MassUnit) as well as str_to_MassUnit function.