fix concatenation issue and copy inputs

[knaaptime]

this fixes the concatenation index issue, and also it turns out we were also modifying inputs as well

[codecov-commenter]

Codecov Report

Merging #80 into master will increase coverage by 3.60%.
The diff coverage is 93.18%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   77.59%   81.19%   +3.60%     
==========================================
  Files          11       11              
  Lines         540      569      +29     
==========================================
+ Hits          419      462      +43     
+ Misses        121      107      -14     

Impacted Files	Coverage Δ
tobler/area_weighted/area_weighted.py	87.28% <88.88%> (+6.03%)	⬆️
tobler/model/models.py	90.00% <88.88%> (-1.18%)	⬇️
tobler/tests/test_interpolators.py	97.95% <100.00%> (+0.13%)	⬆️
tobler/util/util.py	85.71% <100.00%> (+12.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 11e4d09...f27cc7e. Read the comment docs.

[sjsrey]

good catch on the copy.
i think this handles the bug - we need to add the tests.

[knaaptime]

resolves #68 #69 and #6