Implement pdf and cdf for stable distribution

[astralord]

The problems are with

• alpha close to 1 with non-zero beta
• beta close to 0, when alpha = 1
• asymptotic series expansion for alpha = 1 and non-zero beta
• cdf representation for short tails (beta = 1)

[astralord]

Useful papers:
Some improvements in numerical evaluation of symmetric stable
density and its derivatives
Stable distributions and their Applications (Uchaikin and Zolotarev)