Make the AdvancedHandler less magical

[stlehmann]

This PR removes some of the magic from the AdvancedHandler of the AdsTestserver. Namely this is:

1. PLCVariables will need to be added to the handler before they can be written to or read from This gives more control over the creation process and makes it more explicit.

2. PLCVariables need name, value, ads_type and symbol_type on initialisation. This mirrors more the situation we have with a real plc where we create variables upfront with all necessary type informations at hand. Also it brings more clarity in the tests. One can see which values and types are used by simply looking at the test. No need to search for the default values.

[coveralls]

Pull Request Test Coverage Report for Build 668

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.634%

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Totals
Change from base Build 666:	0.0%
Covered Lines:	1164
Relevant Lines:	1230

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 668

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.634%

---

Totals
Change from base Build 666:	0.0%
Covered Lines:	1164
Relevant Lines:	1230

---

💛 - Coveralls