FEATURE: Ability to use agg_lt() without a full life table

[dorwon]

Is your feature request related to a problem? Please describe.
Using agg_lt for aggregating mx and ax is not possible without a complete life table (i.e. a life table containing a row where age_end is Inf). This is because the call of gen_dx_from_lx() does not allow for NA's in the final output, which is inevitable when the input is not a complete life table.

Describe the solution you'd like
Change assert_na in the gen_dx_from_lx() to FALSE. Create a check that determines whether the input contains 1 age group above the oldest age group in the aggregates.

Describe alternatives you've considered
Only allowing full life tables to be used in the function

Additional context
I was working with draw level life tables from multiple countries at the same time. I subset the data to fewer age groups because of potential memory/speed issues.