sprt calculator: allow for specifying alpha and beta parameters

[gahtan-syarif]

generalization/expansion of the sprt calc

not particularly useful, but also wouldnt hurt

fixes contributed by @Viren who merged this pr in his fishtest fork "montytest" https://montychess.org/sprt_calc?elo-model=Normalized&elo-0=0&elo-1=2&draw-ratio=0.49&rms-bias=191&alpha=0.5&beta=0.1

[peregrineshahin]

If the math holds, worth changing it in elo calc api as well

fishtest/server/fishtest/api.py

Line 563 in 12981ff

alpha = 0.05

[peregrineshahin]

changing it to be optional* and not hard-coded.

[gahtan-syarif]

If the math holds, worth changing it in elo calc api as well

fishtest/server/fishtest/api.py

Line 563 in 12981ff

alpha = 0.05

That would depend on if the calculation for the SPRT-adjusted elo is generalized to all alpha/beta values or only 0.05. I think vdbergh would know the answer to that?