Updated to not depend on lalsuite

[duncanmmacleod]

This PR updates gwtrigfind to not depend on lal as follows

• use astropy.time for time conversion
• return basic str URLs instead of CacheEntry

This is a backwards-incompatible change.

[coveralls]

Pull Request Test Coverage Report for Build 7

• 49 of 50 (98.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.3%) to 100.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
gwtrigfind/test_trigfind.py	19	20	95.0%
<!--	Total:	49	50	98.0%	-->

Totals
Change from base Build 65:	2.3%
Covered Lines:	265
Relevant Lines:	265

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💛 - Coveralls