Making basilisk module intermediate variables to be protected

[gjaju313]

Creating new custom modules to add features on top of existing basilisk modules (such as additional biases in body frame and filters to imuSensor module) requires access to its intermediate variables (e.g. omega_PN_P_out for imuSensor) which are declared private.

Declaring them protected will enable using existing basilisk modules as base classes to add features and create custom external modules.

[juan-g-bonilla]

Thank you for the issue, @gjaju313 . This request is usually reasonable, and new code should strive to use protected to facilitate subclassing, like you mentioned. If there are specific models where you'd like to see this change made, feel free to submit a PR!