BitPanda source for XAU/USD and calculate XAU/ERG with CoinCap(combine aggregated sources);

[greenhat]

Close #223

[coveralls]

Pull Request Test Coverage Report for Build 4022694163

• 38 of 74 (51.35%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 61.491%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/main.rs	0	1	0.0%
core/src/datapoint_source/ada_usd.rs	0	2	0.0%
core/src/datapoint_source/aggregator.rs	11	13	84.62%
core/src/datapoint_source/erg_xau.rs	9	13	69.23%
core/src/datapoint_source/bitpanda.rs	9	14	64.29%
core/src/datapoint_source/predef.rs	0	10	0.0%
core/src/datapoint_source.rs	0	12	0.0%
<!--	Total:	38	74	51.35%	-->

Totals
Change from base Build 3994007825:	-0.3%
Covered Lines:	2012
Relevant Lines:	3272

---

💛 - Coveralls