Add support for one-indexing

[blegat]

Copying rowval can be costly so it's better to create is with the right indexing immediately.
With this PR, different indexing are allowed so that for instance SCS can use zero-based indexing as this is what is needed by the solver and DiffOpt can use one-based indexing as it allows it to to an allocation-free conversion to SparseMatrixCSC.

[codecov]

Codecov Report

Merging #22 (54a7483) into master (e039723) will decrease coverage by 1.75%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   85.38%   83.63%   -1.76%     
==========================================
  Files           5        5              
  Lines         308      275      -33     
==========================================
- Hits          263      230      -33     
  Misses         45       45              

Impacted Files	Coverage Δ
src/sparse_matrix.jl	100.00% <100.00%> (ø)
src/change_form.jl	73.97% <0.00%> (-3.14%)	⬇️
src/conic_form.jl	74.11% <0.00%> (-2.73%)	⬇️
src/matrix_input.jl	98.41% <0.00%> (-0.34%)	⬇️

---

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 e039723...2471e47. Read the comment docs.