feature: Add voltage divider device (#492)

* feature: Add voltage divider device

* fix: Rst polish and save metadata

* fix: Save value also when getting

* docs: Better docstring

qcodes/instrument_drivers/__init__.py

@@ -0,0 +1 @@
+from .devices import VoltageDivider

qcodes/instrument_drivers/devices.py

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+from typing import Union
+
+from qcodes import Parameter, StandardParameter
+
+
+class VoltageDivider(Parameter):
+    """
+    Resitive voltage divider
+
+    To be used when you use a physical voltage divider to set or get a voltage.
+
+    Initialize the voltage diveder by passing the parameter to be measured
+    and the value of the division (which should be calibrated beforehand)
+
+    >>> vd = VoltageDivider(dac.chan0, 10)
+
+    The voltage divider acts a your original parameter, but will set the right
+    value, and store the division_value in the metadata.
+
+    Set the value you want to set your device at 10 V
+
+    >>> vd(10)
+
+    This will set the dac.cha0 at 10*10, but upon measuring the divider
+    the value returned is the voltage at the sample.
+
+    >>> vd()
+    10
+
+    To get the voltage that was actually set on the instrument:
+
+    >>> vd.get_instrument_value()
+    100
+
+
+
+    Args:
+        v1: Parameter physically attached to the divider as input
+        division_value: the divsion value of the divider
+        label: label of this parameter, by default uses v1 label
+            but attaches _attenuated
+        name: name of this parameter, by default uses v1 name
+            but attaches _attenuated
+    """
+
+    def __init__(self,
+                 v1: StandardParameter,
+                 division_value: Union[int, float],
+                 name: str=None,
+                 label: str=None) -> None:
+        self.v1 = v1
+        self.division_value = division_value
+        if label:
+            self.label = label
+        else:
+            self.label = "{}_attenuated".format(self.v1.label)
+
+        if name:
+            self.name = name
+        else:
+            self.name = "{}_attenuated".format(self.v1.name)
+
+        super().__init__(
+            name=self.name,
+            instrument=getattr(self.v1, "_instrument", None),
+            label=self.label,
+            unit=self.v1.unit,
+            metadata=self.v1.metadata)
+
+        # extend metadata
+        self._meta_attrs.extend(['v1', 'division_value'])
+
+    def set(self, value: Union[int, float]) -> None:
+        instrument_value = value * self.division_value
+        self._save_val(value)
+        self.v1.set(instrument_value)
+
+    def get(self) -> Union[int, float]:
+        """
+        Returns:
+            number: value at which was set at the sample
+        """
+        value = self.v1.get() / self.division_value
+        self._save_val(value)
+        return value
+
+    def get_instrument_value(self) -> Union[int, float]:
+        """
+        Returns:
+            number: value at which the attached paraemter is (i.e. does
+            not account for the scaling)
+        """
+        return self.v1.get()