Introduce BMNT and EENT getters

In a military context, BMNT and EENT are used; Begin Morning Nautical Twilight (i.e., Nautical Dawn) and End Morning Nautical Twilight (i.e., Nautical Dusk).

Even though this would be a slight duplicate/split of SunriseSunset.getNauticalTwilight(), consider adding the following two methods, as a military context application would probably only use one or the other method per situation:

.getBmnt();
.getEent();