89 add spawn hom with examples

[TobiasSpratte]

This PR closes #89. We added the Spawn model corresponding to the High Order Model (HOM) contained in the AixLib library to BESMod. Furthermore, we added two example models (CalcHeaDemHOMSpawn & HOMSpawn). Possible errors and solutions when using the Spawn model were added to the documentaion of the Spawn model.

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