feat(eclipse): add getSolarSarosNumber

[michealroberts]

To calculate the saros and inex numbers of an eclipse in every eclipse season from 11,000 BC to AD 15,000 based on a given approximate date:

First, calculate the index of the eclipse season as:

const index = Math.floor((datetime + 2882.55) * 2.1074515 + 0.5);

where "datetime" is the number of Gregorian years since January 1, 1 BC in the proleptic Gregorian calendar.

Next, calculate how many times a five-month period separates two targeted eclipses using the formula:

const N5 = Math.floor(index / 7.62263 + 1.024 
        - Math.pow(index / 2.107452 / 17200, 2) 
        - Math.pow(index / 2.107452 / 22300, 3) 
        + 0.09 * Math.sin((index - 0.79) / 0.3354265));

The saros number for the eclipse is then:

const saros = 5 * index - 38 * N5;

and the inex number is:

const inex = 8 * index - 61 * N5;

To calculate the saros and inex numbers for a lunar eclipse near a given date, add nine years to the date and find the numbers for the solar eclipse half a saros later, then subtract 7 from the solar saros number to obtain the lunar saros number, and subtract 13 from the solar inex number to obtain the lunar inex number.