lt_abridged unit tests

[timriffe]

• [ ] transitivity (dx, lx, qx all agree, if mx derived from qx, then a lifetable from mx or qx should be identical). should only have perfect transitivity of axmethod = "un".
• [ ] check variable bounds. (no neg mx)
• [ ] check age classes
• [ ] check that defaults working properly for old age mortality. currently extrapLaw makeham if max(Age) <90, otherwise kannisto. Check that extrapFit is respected. Ensure messages sent to console when a default is set.
• [ ] check that 0s don't break things! a(x) should still be reasonable.
• [ ] high nMx shouldn't cause breaks
• [ ] warn if M(80) < M(70)
• [ ] check that all user choices are respected.
• [ ] detect if users don't separate infants. In this case, we should split it for them and warn. Choice 1) lt_model_lq() Choice 2) use t_rule_4m0_m0() and some derivations. Implement, catch, and warn :-)