Replace bitUSD reference value with BTS:USD basket

[grctest]

Currently BSIP42 poses a potential threat to the accuracy of bitUSD's settlement price - a value which we reference in some of the hertz price feed scripts:

market = Market("USD:BTS") # Set reference market to USD:BTSL29

We need an alternative USD reference than bitUSD, ideally it would sample multiple BTS trading pairs (which aren't affected by BSIP42) to estimate the value of USD in BTS.

[grctest]

BSIP 76 (bitshares/bsips#221) has now set fixed (fake) price feeds for bitUSD.

Price feed scripts which use bitUSD as the price feed reference are now producing inaccurate Hertz feeds.

An alternative to bitUSD must be found for Hertz to become accurate again.

[grctest]

The internal scheduled price feed script has now added coingecko as an alternative source of BTS/USD reference: dd433b7