fix(slice): support `slice 1 _`

[grddavies]

fixes: #979

Slice was failing on any slice pN of 1. This was because the begin of the next slice was being used to calculate the end of the first. With slice 1 _ all slices begin at 0.0, so end is incorrectly set to 0.0.

This implementation calculates the end control pattern by generating a sample-width pattern, and adding this to the begin pattern.

Two tests are added:

• slice "can slice samples" - Regression test that passes on dev and this branch
• slice "can slice by 1" - Test for accurate calculation of sample end time when slicing into 1

[yaxu]

Thanks!