Adding a PNA driver (untested).

Slight modification in InstrTask and Roottask to store driver in dict and never create two drivers for the same instrument. Slight alteration of instrument property to avoid setting a value if it is not necessary.
Some small modif and new tasks for the PNA.
Fixing bugs in Pna drivers and tasks. (underwork)
Adding driver and task for Oxford magnet.
Cleaning a lot of things :

- new function to format replacement string so that all math notation are defined
- systematic use of field replacement in target value for instrument
- use of replacement fields for loops
- creation of a simple loop task with no children task
- suppression of timing loop replaced by an option in all loop tasks
- new filter for loop tasks
- save task should now write its header
Small work ......
Going on debuging the PNA driver
Fixing small things (bug in meas_exec). Changing some names and names formatting. Adding a driver for Anritsu signal generator
Adding the possibility to specify an attribute to control make_parallel and make_wait. (Useful for extra fast loop). Modifying the task_database_entries trait to be a dictionary so that one can give the default value easily.
Huge clean up.

- Changed make_wait and make_parallel to accept argument to bypass them
- full debug of all tasks non instr
- full debug of instr task save PNA and mag_field
......
Adding to the list of the Array Extrema task and debugging it (works).
Merge branch 'Doc'