Use AlphaSimR::newEmptyPop() to simplify some of SIMplyBee code

[gregorgorjanc]

When we started developing SIMplyBee we have hit some edge cases with empty AlphaSimR Pop objects. We have worked around that with some ugly & convoluted R code, but made it work eventually.

I now see that AlphaSimR has newEmptyPop(), which could simply some of the ugly & convoluted R code.