Adds asymmetric SAM data (clean PR)

[eliocamp]

This adds the asymmetric and symmetric SAM indices.

I called the functions "asymsam", from "asymmeric SAM", but let me know if you want me to change them.

library(rsoi)

download_asymsam_monthly() |> 
  head()
#>   Lev Index       Date       Value Value_normalized
#> 1   1   sam 2023-09-01  0.05177943       0.14952028
#> 2   1  ssam 2023-09-01  0.43155471       0.06490490
#> 3   1  asam 2023-09-01 -0.37977528       0.21935814
#> 4   2   sam 2023-09-01  0.04575554       0.22199278
#> 5   2  ssam 2023-09-01  0.54510591       0.09215533
#> 6   2  asam 2023-09-01 -0.49935037       0.24345311


download_asymsam_daily(levels = c(700, 500)) |> 
  head()
#> Downloading level: 700
#> Downloading level: 500
#>   Lev Index       Date       Value    R.squared
#> 1 700  asam 1940-01-01 -0.13508871 0.0426478529
#> 2 700   sam 1940-01-01 -0.12471444 0.0054371573
#> 3 700  ssam 1940-01-01  0.01037427 0.0000442406
#> 4 700  asam 1940-01-02 -0.11471232 0.0345327849
#> 5 700   sam 1940-01-02  0.02685265 0.0002830512
#> 6 700  ssam 1940-01-02  0.14156497 0.0092506011

^{Created on 2023-10-30 with reprex v2.0.2}

Closes #47

[boshek]

Thank you for this!