FT: Modify Assertion checking + Assumptions prototype

[shamindras]

This is per #60

Assertion Checking fixes

Per discussion we should implement the following:

• Update assertion checking for get_summary() and summary()
• Update assertion checking for get_confint() and confint()
• Check whether we need to update assertion checking for print()

Update tibble based assumptions

• @shamindras : Setup a prototype of get_assumptions() to use tibbles
• @shamindras : Do the same for the variance titles
• @shamindras : Do the same for the variance emojis
• @shamindras : Ensure that this flows through to comp_var() and it contains only tibbles
• @shamindras : Ensure that the flow through to other parts of the code is easier using the revised tibbles

Quick questions

• Do we need this line? If not, we should just remove it in a commit