Add <default_value> Handling to ROS_PARAM_SUBSTITUTIONS, fix types

(Potentially) Breaking Changes

• Added requirements.txt with rospkg and pyyaml (these were previously required, but not in a requirements file.
• Substitutions now have the correct type (they aren't always strings)

Improvements

• Added default value handling in rosparam subsitution: ${rosparam_name; <default_value>} (caveat: MUST use semicolon to allow list, dict substitutions)

Bugfixes

• Substitutions now have the correct type (they aren't always strings)