Provide option to avoid limiting atmospheric emissivity to 1.0

Providing an optional function argument to blkBody_Emissivity(...) to determine whether or not the upper limit of 1.0 should be enforced for the calculated emissivity.

The air emissivity is needed by SNOWPACK for the linearization of the energy balance. But the hard-coded upper limit of 1.0 leads to discrepancies where ILWR is calculated to EA, and then back-calculated to ILWR. This can result in a markedly different ILWR in the output than was provided in the input data. Having this optional argument would allow to avoid this in SNOWPACK.