Changed references in the Strategy objects of the backtesters to 'per…

[haxwell]

Changed references in the Strategy objects of the backtesters to 'periodLength' rather than 'period'.

[Fredrik81]

Hello,
After merge of this i get a new error:
zenbot/zenbot/scripts/auto_backtester/backtester.js:109
let rawParams = jsonRegexp.exec(output2)[1];

TypeError: Cannot read property '1' of null
at processOutput (/zenbot/scripts/auto_backtester/backtester.js:109:47)
at shell.exec (/zenbot/scripts/auto_backtester/backtester.js:94:14)
at /zenbot/node_modules/shelljs/src/exec.js:199:9
at ChildProcess.exithandler (child_process.js:267:7)
at emitTwo (events.js:126:13)
at ChildProcess.emit (events.js:214:7)
at maybeClose (internal/child_process.js:925:16)
at Socket.stream.socket.on (internal/child_process.js:346:11)
at emitOne (events.js:116:13)
at Socket.emit (events.js:211:7)

[haxwell]

@Fredrik81 I think you'll find that you got that same error before my period/periodLength changes.. this PR just gets you back to that state :)

In other words, its a separate issue.