Nearest neighbor interpolator

[zihaoxu98]

This PR adds nearest neighbor interpolator, in case where the inverse distance weighting (IDW) does not make sense any more.

To use it, add method="NN" in the Map. For example,

apt.config.Map(
    name='test_map',
    default='map_file.json',
    method='NN',
)

[coveralls]

Pull Request Test Coverage Report for Build 9483108512

Details

• 19 of 45 (42.22%) changed or added relevant lines in 2 files are covered.
• 23 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.7%) to 85.04%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
appletree/config.py	11	17	64.71%
appletree/interpolation.py	8	28	28.57%

Files with Coverage Reduction	New Missed Lines	%
appletree/config.py	23	83.4%

Totals
Change from base Build 9481588762:	-0.7%
Covered Lines:	2359
Relevant Lines:	2774

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9483112472

Details

• 20 of 44 (45.45%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 85.04%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
appletree/config.py	12	16	75.0%
appletree/interpolation.py	8	28	28.57%

Totals
Change from base Build 9481588762:	-0.7%
Covered Lines:	2359
Relevant Lines:	2774

---

💛 - Coveralls

[dachengx]

elif len(self.coordinate_lowers) == 3 and self.method == "IDW":

to

elif len(self.coordinate_lowers) == 3:

[dachengx]

#170