feat: plotting ncl densities (#120)

* add kinetic energy density
* return density in different format for kinetic energy density

* plot ncl magnetization
* make plotting work for more densities
* fix plots for non standard-cell ystems
* allow multiple isosurfaces in single plot

* add to_numpy function

* add selections for source to all classes
* improve documentation for selections
* selection for quantity and components with synonyms for easy access
* improve documentation of Density.selections() to see available options

* implement aliases for density
* format sources that are aliases different in YAML output

---------

Co-authored-by: Marie-Therese Huebsch <marie-therese.huebsch@vasp.at>
Co-authored-by: Martin Schlipf <martin.schlipf@gmail.com>

.gitignore

@@ -1,3 +1,4 @@
+*~
 # File open for editing in vi
 *.swp
 # autogenerated by dephell

src/py4vasp/_data/base.py

@@ -122,6 +122,10 @@ def path(self):
     def _raw_data(self):
         return self._data_context.data
 
+    @property
+    def _selection(self):
+        return self._data_context.selection
+
     @data_access
     def print(self):
         "Print a string representation of this instance."
@@ -131,6 +135,23 @@ def read(self, *args, **kwargs):
         "Convenient wrapper around to_dict. Check that function for examples and optional arguments."
         return self.to_dict(*args, **kwargs)
 
+    @data_access
+    def selections(self):
+        """Returns possible alternatives for this particular quantity VASP can produce.
+
+        The returned dictionary contains a single item with the name of the quantity
+        mapping to all possible selections. Each of these selection may be passed to
+        other functions of this quantity to select which output of VASP is used.
+
+        Returns
+        -------
+        dict
+            The key indicates this quantity and the values possible choices for arguments
+            to other functions of this quantity.
+        """
+        sources = list(raw.selections(self._data_context.quantity))
+        return {self._data_context.quantity: sources}
+
     def _repr_pretty_(self, p, cycle):
         p.text(str(self))
 
@@ -308,4 +329,5 @@ def __exit__(self, *_):
             self._stack.close()
 
     def set_selection(self, selection):
-        self.selection = selection
+        if self._counter == 0:
+            self.selection = selection