Make AdsSymbol even more pythonic

[stlehmann]

The purpose of this PR is to make the great AdsSymbol class even more Pythonic by:

• replacing setter/getter functions by properties
• Allowing immediate writing of the value to the plc if auto_update is True

[coveralls]

Pull Request Test Coverage Report for Build 556

• 26 of 26 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 94.401%

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Totals
Change from base Build 544:	0.06%
Covered Lines:	1096
Relevant Lines:	1161

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💛 - Coveralls

[stlehmann]

@RobertoRoos I made some minor API-changes to make AdsSymbol class more pythonic. You might want to have a look at it and tell me if you approve it.

[RobertoRoos]

I think if self.auto_update it's a little easier to read.

[stlehmann]

That's a good suggestion. I implemented the auto_update property after the value property so I didn' t see this. I will change it.

[RobertoRoos]

I am personally not the biggest fan of magic properties, but I understand this is considered good practice.

Placed a teeny-tiny comment, I like your changes. Thank you for asking me.

[stlehmann]

@RobertoRoos thanks for having a quick look at it :)