Replace filters by libdlf

[prisae]

As libdlf exists, outsource the filters to lib.

Pros

• Reduce significantly the code size of empymod (removes over 50% of loc).
• Should be faster due to caching of filters.
• Filters only in one place.
• Future filters can come with libdlf; no need for empymod to change.

Contra

• Adds an additional dependency (but super lightweight).

TODO's

• [x] Implement loading from libdlf and caching
• [x] Deprecate old way
• [x] Adjust documentation everywhere!
• [x] Adjust internal usage
• [x] Adjust tests
• [x] Adjust examples
• [x] Add tests for new things

Closes #136

[coveralls]

Pull Request Test Coverage Report for Build 8116212556

Details

• 63 of 73 (86.3%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 58.389%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
empymod/model.py	0	4	0.0%
empymod/filters.py	51	57	89.47%
<!--	Total:	63	73	86.3%	-->

Totals
Change from base Build 8046551655:	0.5%
Covered Lines:	3334
Relevant Lines:	5710

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💛 - Coveralls