Triplet design matrix speedup

[scottstanie]

Description of proposed changes

Speeds up the creation of the closure phase matrix in ifgramStack.get_design_matrix4triplet (discussed in #649). Should result in same output as previous implementation

Reminders

• discussed in speeding up get_design_matrix4triplet? #649
• Pass Codacy code review (green)
• Pass Circle CI test (green)
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[yunjunz]

Looks great, thank you @scottstanie!