Tahigash ncdiff sync and enhs

[tahigash]

* yang.connector
    * updated `request` method
* yang.ncdiff
    * synced PR2, 3, 5, 8, 13, 14, 19, 21, 24, 25, 26 and 27 from ncdiff repository
    * updated NetconfCalculator class
        * added diff_type `minimum-replace`
        * added `add_attribute_at_depath` method
        * added `add_attribute_by_xpath` method
        * added `find_by_tags` method
    * updated ConfigDelta class
        * added `replace_xpath` argument
        * added `find_by_tags` method

NOTE: codes syncing from ncdiff are reviewed and merged on ncdiff repo. so no need to review those much.
https://github.com/CiscoTestAutomation/ncdiff/pulls?q=is%3Apr+is%3Aclosed

connector UT:

Ran 16 tests in 0.898s

FAILED (errors=1)

NOTE: same error even with master branch. so not because of my PR

ncdiff UT:

Ran 54 tests in 0.291s

OK

[dwapstra]

Code looks fine, please update docs as commented.