Read signature files with missing pings.

[lkilcher]

Fix for reading signature files that having missing pings

[coveralls]

Pull Request Test Coverage Report for Build 1763993653

• 95 of 114 (83.33%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 88.942%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dolfyn/velocity.py	2	3	66.67%
dolfyn/tests/test_read_adp.py	13	16	81.25%
dolfyn/time.py	8	14	57.14%
dolfyn/io/nortek2_lib.py	54	63	85.71%

Files with Coverage Reduction	New Missed Lines	%
dolfyn/tests/test_read_adp.py	1	77.3%

Totals
Change from base Build 1763926864:	-0.1%
Covered Lines:	4440
Relevant Lines:	4992

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💛 - Coveralls