FT: Fix get_confint() and confint() and assertion checking

[shamindras]

This is to be done after #55 is fully completed and merged in #56 .

get_confint() and confint() related

• Write prototype of get_confint() tibble with pivot_longer() working with the confidence level. Ensure that this is a valid tidy format for this table. Only conf.low and conf.high have the level parameter as not an NA value
• Get prototype of get_confint() tibble with pivot_wider() working with the confidence level
• Fix get_confint() to return conf.low/conf.high return levels
• Fix confint to use get_confint() and group_split as discussed [here](https://maarshq.slack.com/archives/D01JL7EAW64/p1615811868000700?thread_ts=1615777275.013500&cid=D01JL
• Fix confint to print levels

Assertion Checking

• @shamindras : Fix assertion handling for lm only, maars_lm, maars_glm, glm only. This will replace code like this
• Do assertion handling for missing parameters e.g. running just lm_fit %>% maars::comp_var() with no parameters, and handle this elegantly