Fix spread

[zbuc]

Seems like spread is lower than we'd expect:

 ID                                                                Trading Pair           State   Reserves                                                  Trading Function
 plpid17syw9sl0mwsrr482hxf7y9tmh0jv7w6hs6277nzv80rwe8a636kqwnl8k7  (test_usd, test_atom)  opened  (1475645.79296386000351049, 3333129.882812)               fee: 7bps, p/q: 0.000000, q/p: 10904000000000.000000000000000000
 plpid1kjqpfh7ua7epfdxr2pdzvut3ne0w66kdf4nxnjsnyjqklsy2dj5qs5faym  (test_btc, test_eth)   opened  (1995754.39095497, 1991842.091083526611328125)            fee: 0bps, p/q: 151515151515.151520, q/p: 0.000000000006600000
 plpid1lzw82fhhtdfccj77lxc7hq7axz0u9awajvje8auqda8j9z3mn40qgz9ugh  (test_btc, test_usd)   opened  (2005718.42491626, 1411875.357335929948021658)            fee: 2bps, p/q: 269594400000000.000000, q/p: 0.000000000000003709
 plpid1uqz2wee8cjnrpnutepxuj4r8kmtdz7tjy9kc3u0952pm0vwr6kzqhews9f  (test_usd, test_eth)   opened  (1507059.578293592494446784, 1994176.059961318969726562)  fee: 3bps, p/q: 0.000557, q/p: 1795.420000000000072760

[zbuc]

The spreads need to be multiplied by 2, currently they are only reflecting the distance between the midpoint and best bid.

 ID                                                                Trading Pair           State   Reserves            Trading Function
 plpid17mp92jyulqfvrzcf70ufyt8zaz58yx8vv70n4x49n2h2x0q2z9uq5y6jg3  (test_btc, test_atom)  opened  (1250000, 2500000)  fee: 29bps, p/q: 24.509804, q/p: 0.040800000000000003
 plpid17scedn6mclhh5q62llnyzwh0hwwyqvge6smpneakytjdzzzzmlmsc97h4p  (test_usd, test_eth)   opened  (156250, 625000)    fee: 1bps, p/q: 0.000555, q/p: 1800.339999999999918145
 plpid19v7le8kah45gzzrehmuffqgkyhv4ewm4egw9lmv8yrqhlxtjtjhqcfdl02  (test_btc, test_usd)   opened  (625000, 312500)    fee: 1bps, p/q: 270577500000000.000000, q/p: 0.000000000000003696
 plpid1hhepyuzrvv88mx42d5gxr7gengllsawh4hpfmefq8atd7wulxens2hsf3y  (test_usd, test_atom)  opened  (2578125, 3125000)  fee: 7bps, p/q: 0.000000, q/p: 11054000000000.000000000000000000
 plpid1wwxn0lyp03pkwhjj5h4gjlzmlga6y0zfvt606feucqh63gfka06qxkkmwa  (test_btc, test_eth)   opened  (312500, 1250000)   fee: 4bps, p/q: 151515151515.151520, q/p: 0.000000000006600000

For the test_usd, test_eth pair:

>>> best_bid = 1799.91000000
>>> best_ask = 1800.44000000
>>> midpoint = (best_bid + best_ask)/2.0
>>> midpoint
1800.1750000000002
>>> (midpoint - best_bid) / midpoint
0.00014720791034210563
>>> ((midpoint - best_bid) / midpoint) * 100 * 100
1.472079103421056
>>> ((best_ask - best_bid) / midpoint) * 100 * 100
2.9441582068408496