HeatIndex for lower values of temperature and humidity

The current algorithm used for calculating heat index takes into account Rothfusz equation to achieve the values that Robert G. Steadman came up by experimentation. It then has some refinements done by the NWS but doesn't consider the other refinements that NWS did for lower values of temperature and humidity.

That's the reason why heat index for common house-hold values, like 74.84F and 68.5H give a heat index of 66.38F when in reality it should be somewhere in the range of 75.23F - which makes more sense.

NWS full algo is very well detailed in this paper:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3801457/

I also submited a pull request:
https://github.com/adafruit/DHT-sensor-library/pull/154

Review the current heat-index calculation in the library and compare it with the NWS algorithm described in the linked paper, focusing on lower temperature and humidity ranges. The work is complete when common household inputs such as 74.84F and 68.5H produce a heat index near the reported 75.23F range; a pull request for this issue is already linked in the report.

Written by the indexing model from the issue text.