[BUG] Behavior difference between PMD and OpenDSS for transformers with xhl=0 xht=0 xlt=0

[John-Boik]

In the OpenDSS test file ut_trans_3w_dyy_3.dss, and perhaps others, parameters for a transformer (TX1) include:

xhl=0 xht=0 xlt=0

Parsing the file with PMD results in values "xsc" => Union{Missing, Float64}[0.0, 0.0, 0.0]. However, loading the file via OpenDSSDirect results in "xsc" => [7.0, 35.0, 30.0]. I originally thought this was a bug in OpenDSSDirect and filed Issue 101, but the response there informed me that defaults are used when any of xhl=0 xht=0 xlt=0. As such, the results of PMD circuit analysis can be expected to differ from that of OpenDSSDirect. Is this PMD behavior desired and expected?