write configured parameters to the hardware

writable parameters with a configured value should call write_<param>
on initialization.
+ introduced 'initwrite' parameter property for more fine control over this
+ minor improvements in metaclass.py, param.py, commandhandler.py
+ rearranged test_modules.py

Change-Id: I2eec45da40947a73d9c180f0f146eb62efbda2b3
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21986
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

test/test_params.py

@@ -27,6 +27,7 @@ import pytest
 
 from secop.datatypes import BoolType, IntRange
 from secop.params import Command, Override, Parameter, Parameters
+from secop.errors import ProgrammingError
 
 
 def test_Command():
@@ -55,7 +56,7 @@ def test_Parameter():
     p2 = Parameter('description2', datatype=IntRange(), constant=1)
     assert p1 != p2
     assert p1.ctr != p2.ctr
-    with pytest.raises(ValueError):
+    with pytest.raises(ProgrammingError):
         Parameter(None, datatype=float)
     p3 = p1.copy()
     assert p1.ctr != p3.ctr