calc_stochrsi: Stochastic RSI (StochRSI) for multiple price series
Description
Computes Stochastic RSI (%K) per column over a rolling window, returning
values in [0, 1]. For each symbol, RSI is computed with TTR::RSI() over
rsi_length periods; then StochRSI is
\((RSI_t - \min RSI_{t-L+1:t}) / (\max RSI_{t-L+1:t} - \min RSI_{t-L+1:t})\),
where \(L\) is stoch_length. If the range is zero the value is handled
per on_const_window (default "zero").