AMI430 driver: safety around coil_constant and ramp limits

Nobody has claimed this yet.

I want to change the physical coils of the magnet, and I would like to update the safety limits accordingly.

I use the code:
```
ami_yellow.current_ramp_limit(0.1762) # A/s
ami_yellow.ramp_rate(0.1762 * 0.01490) # T/s <<<-- exception is raised here
ami_yellow.coil_constant(0.01490) # T/A
ami_yellow.current_limit(67.11) # A
```

(unfortunately traceback is lost, but the exception originated from `_set_ramp_rate` method, it's the ValueError that that method may raise (see line 369 of AMI430.py).)

It seems that the reason for this exception is that field_ramp_limit was not updated when I set current_ramp_limit above. Hence, the driver thinks that my ramp rate is out of bounds even though it is not (because i set the current_ramp_limit before).

Then, i tried the following:
```
ami_yellow.current_ramp_limit(0.1762) # A/s
ami_yellow.field_ramp_limit(0.1762 * 0.01490) # T/s <<<-- NOTE that I ALSO set field_ramp_limit parameter this time
ami_yellow.ramp_rate(0.1762*0.01490) # T/s
ami_yellow.coil_constant(0.01490) # T/A
ami_yellow.current_limit(67.11) # A
```

And this time there was no exception raised.

However, setting field_ramp_limit seems to overwrite my current_ramp_limit (because coil constant has not updated yet, i.e. it is not "0.01490" yet but some other previous value), so current_ramp_limit apparently gets set to a value which is out of my safety limits.

My proposed solution is a method of setting all safety limits and constants in a single function, and sending those to the instrument in a single call, preferebly in __init__.
Moreover, I would even propose to make the coil_constant, current_ramp_limit (and thereby field_ramp_limit) and current_limit to unchangable after __init__ (so that when coils are changed, the user has to reinstantiate the magent driver).

Along the way found the following bug:
* scale of field_ramp_limit (which is dependent on the coil constant) is fixed in __init__, and this will bring problems if users change coil constanst.

Review AMI430.py, especially __init__ and _set_ramp_rate around line 369, alongside the coil_constant, current_ramp_limit, field_ramp_limit, and current_limit setters. Confirm how these values interact when configuring a changed coil, then define completion as safety limits remaining consistent without the reported exception or unintended overwrite.

Written by the indexing model from the issue text.