Feature/filter data

[dfortem]

This PR is supposed to allow the user to read only a partial amount of the data variable instead of reading it all first and filter later.

There are 2 solutions to do so:
Either you read consequent data and then you just Slice the variable data: getDataVariableSlice(variableName, startIndex, size)
Or you filter according to all dimensions: getDataVariableFiltered(variableName, positionX, sizeX, positionY, sizeY...)

I also added some tests.

[codecov]

Codecov Report

Merging #15 into master will decrease coverage by 75.65%.
The diff coverage is 11.11%.

@@             Coverage Diff             @@
##           master      #15       +/-   ##
===========================================
- Coverage   93.04%   17.39%   -75.66%     
===========================================
  Files           5        5               
  Lines         230      299       +69     
  Branches       40       57       +17     
===========================================
- Hits          214       52      -162     
- Misses         16      247      +231

Impacted Files	Coverage Δ
src/index.js	16.27% <10.52%> (-83.73%)	⬇️
src/data.js	14.75% <13.33%> (-85.25%)	⬇️
src/header.js	10.1% <3.12%> (-84.9%)	⬇️
src/utils.js	47.05% <66.66%> (-52.95%)	⬇️
src/types.js	22.78% <0%> (-62.03%)	⬇️

---

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 32d5e2a...b478309. Read the comment docs.