resample for windows

Problem

1. each window should start fitting at an azimuth that is consistently spaced from the prev window.
2. this spacing is determined by samplerate and initial azimuth in Win(1).
3. so Win(i) should always know where Win(1).va(1) is.

Solution

1. Win(1).va(1) should be saved somewhere.
2. Condition 1: Win(i).a_start = Win(1).va(1) + N * a_per_samp.
3. Condition 2: Win(i).a_start >= Win(i).va(1) + LEN_WIN