[Bug] ta/bbands: when value contains a decimal: error: argument -s/--std: invalid check_positive value: '4.5'

[soggyomelette]

Description

Fixes #1978 by switching type check from 'check_positive' to 'check_positive float'
Before:
(🦋) /crypto/ta/ $ bbands -s 4.5
usage: bbands [-l N_LENGTH] [-s N_STD] [-m S_MAMODE] [-h] [--export EXPORT]
bbands: error: argument -s/--std: invalid check_positive value: '4.5'

After:

Also fixes #1977 by switching labels
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[DidierRLopes]

Nice one @soggyomelette 😄