Add docs, tests for data interface

Documenting how to add data to ICARTT file in docs and usage_examples. @FObersteiner could you review this and report back if this is sufficient and makes sense to users?

src/icartt/dataset.py

@@ -30,6 +30,8 @@ class VariableType(IntEnum):
 
 
 class DataStore1001:
+    """Data model for FFI1001"""
+
     def __init__(self, ivar, dvars):
         self.ivarname = ivar.shortname
 
@@ -111,6 +113,8 @@ class DataStore1001:
 
 
 class DataStore2110(collections.UserDict):
+    """Data model for FFI2110"""
+
     def __init__(self, ivar, ibvar, auxvars, dvars):
         self.ivarname = ivar.shortname
         self.ibvarname = ibvar.shortname
@@ -162,6 +166,19 @@ class DataStore2110(collections.UserDict):
             self.data[ivarValue] = {"AUX": auxds, "DEP": depds}
 
     def add(self, newAuxData, newDepData):
+        """(bulk) add data, providing (structured) numpy arrays for both the auxiliary and dependent data line(s)
+        for a given ivar value.
+
+        Arrays have to have shape [ (ivar, auxvar, auxvar, ...) ] and
+        [ (ibvar, depvar, depvar, ...), ... ] for auxiliary and dependent data line(s), respectively.
+        missing values have to be set to np.nan.
+
+        :param newAuxData: auxiliary data line to be added
+        :type newAuxData: numpy.ndarray
+
+        :param newDepData: auxiliary data line(s) to be added
+        :type newDepData: numpy.ndarray
+        """
         auxds = DataStore1001(self.ivar, self.auxvars)
         depds = DataStore1001(self.ibvar, self.dvars)
 
@@ -424,7 +441,12 @@ class Dataset:
         if self.defineMode:
             return np.datetime64("NaT")
 
-        if self.data.data is None or self.data.data == {}:
+        # for 1001, its an array, for 2110 a dict
+        if not isinstance(self.data.data, (np.ndarray, dict)):
+            return np.datetime64("NaT")
+
+        # it is possible to have an empty dict
+        if isinstance(self.data.data, dict) and not self.data.data:
             return np.datetime64("NaT")
 
         ref_dt = np.datetime64(datetime.datetime(*self.dateOfCollection), "ns")