Add pool_sd option in pairwise_ttests

[raphaelvallat]

This is the default behavior of pairwise.t.test and JAMOVI. Can be manually specified in JASP.

[raphaelvallat]

This should apply to:

• Mixed design (between-subject contrast + interaction)
• 2-way factorial design (main effects + interaction)
• 2-way repeated measures design (interaction only)