Changes

Rewrite using external functions from ModelicaTime.c

* Rewrite DateTime.'constructor'.fromEpoch to utilize ModelicaTime_localtime
* Rewrite DateTime.'String'.formatted to utilize ModelicaTime_strftime
* Rewrite DateTime.epoch to utilize ModelicaTime_difftime
* Add DateTime.'constructor'.fromString utilizing ModelicaTime_strptime

ModelicaTime.[ch] need to be integrated to ModelicaExternalC for later distribution. During development ModelicaTime.c is directly included by the external function declaration.