[Bug] `stocks/ta/bbands` --std argument does not work.

[deeleeramone]

Instead of 2 standard deviations, some users may want to use a 1.5 STD, which fails a check_positive_value test.

/stocks/ta/ $ bbands --std 1.5

usage: bbands [-l N_LENGTH] [-s N_STD] [-m {ema,sma,wma,hma,zlma}] [-h] [--export EXPORT] [--sheet-name SHEET_NAME [SHEET_NAME ...]]
bbands: error: argument -s/--std: invalid check_positive value: '1.5'

Setting --std at 1 does not change the value from the default setting of 2.

The same applies when the value is set at 4.