Add session kwarg to ModelicaSystem constructor

If the session kwarg is specified, the ModelicaSystem will use that
session instead of creating a new one. This is useful for creating a
ModelicaSystem for a model that is already loaded in an already existing
session.

Remove __del__ method from ModelicaSystem. Having the deleter there
meant that the deleter for the session would run multiple times if a
session had shared ownership. The deleter will still run when the
session's reference count reaches zero, or when it is garbage collected.

Author: AxelMatstoms

OMPython/__init__.py

@@ -821,7 +821,7 @@ class ModelicaSystem(object):
     def __init__(self, fileName=None, modelName=None, lmodel=None,
                  useCorba=False, commandLineOptions=None,
                  variableFilter=None, customBuildDirectory=None, verbose=True, raiseerrors=False,
-                 omhome: str = None):  # 1
+                 omhome: str = None, session: OMCSessionBase = None):  # 1
         """
         "constructor"
         It initializes to load file and build a model, generating object, exe, xml, mat, and json files. etc. It can be called :
@@ -831,11 +831,14 @@ def __init__(self, fileName=None, modelName=None, lmodel=None,
         Note: If the model file is not in the current working directory, then the path where file is located must be included together with file name. Besides, if the Modelica model contains several different models within the same package, then in order to build the specific model, in second argument, user must put the package name with dot(.) followed by specific model name.
         ex: myModel = ModelicaSystem("ModelicaModel.mo", "modelName")
         """
+        if session is not None:
+            self.getconn = session
+        elif useCorba:
+            self.getconn = OMCSession(omhome=omhome)
+        else:
+            self.getconn = OMCSessionZMQ(omhome=omhome)
+
         if fileName is None and modelName is None and not lmodel:  # all None
-            if useCorba:
-                self.getconn = OMCSession(omhome=omhome)
-            else:
-                self.getconn = OMCSessionZMQ(omhome=omhome)
             return
 
         self.tree = None
@@ -857,11 +860,6 @@ def __init__(self, fileName=None, modelName=None, lmodel=None,
 
         self._verbose = verbose
 
-        if useCorba:
-            self.getconn = OMCSession(omhome=omhome)
-        else:
-            self.getconn = OMCSessionZMQ(omhome=omhome)
-
         ## needed for properly deleting the OMCSessionZMQ
         self._omc_log_file = self.getconn._omc_log_file
         self._omc_process = self.getconn._omc_process
@@ -906,9 +904,6 @@ def __init__(self, fileName=None, modelName=None, lmodel=None,
 
         self.buildModel(variableFilter)
 
-    def __del__(self):
-        OMCSessionBase.__del__(self)
-
     def setCommandLineOptions(self, commandLineOptions: str):
         ## set commandLineOptions if provided by users
         if commandLineOptions is not None: