calculate intermediate throws

[helbling]

calculate intermediate throws to help start the pattern
let the user choose between different ways to start / end the pattern

[helbling]

so far there is only few example of intermediate throws people actually do:

• start with up to three 4s to swap straight/crossing and/or start lefthanded
• replace 2 with 4 to avoid zipping into full hand. examples: 972 starting 974 and 92978 starting 94978