update/document how to change Configuration object parameters about desisurvey

[dkirkby]

The currently supported method to change a config param in memory is:

  c.first_day._value = <DATE>

However, this operation should not normally be necessary (except, e.g. for specialized testing code) so is reserved for experts. Therefore I consider it a feature, not a bug, that the following does not work:

  c.first_day = <DATE>

I could hide the _value attribute by adding a method, e.g.

  c.first_day.set_value(<DATE>)

but wanted to check back with you to see your current thinking first.

[sbailey]

OK for c.first_day.set_value(<DATE>) if you prefer that over c.first_day = <DATE>.

Even if it is primarily to support simulations/verification/testing, we should have a way to override a config file value that doesn't carry the implied "this might break later" message of telling users to use a private _value variable.

[dkirkby]

The new set_value() method is implemented now in #66, with accompanying docstring and unit test updates. I also updated earlier instances of _value = X to use set_value(X).