ENH: full/proper support for multi-dimensional BasisParams

[sglyon]

closes #45

[coveralls]

Changes Unknown when pulling 1ec67ab on sl/multidim_params into ** on master**.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@9fd30c6). Click here to learn what that means.
The diff coverage is 93.93%.

@@            Coverage Diff            @@
##             master      #46   +/-   ##
=========================================
  Coverage          ?   92.63%           
=========================================
  Files             ?       12           
  Lines             ?     1385           
  Branches          ?        0           
=========================================
  Hits              ?     1283           
  Misses            ?      102           
  Partials          ?        0

Impacted Files	Coverage Δ
src/cheb.jl	96.11% <ø> (ø)
src/BasisMatrices.jl	92.3% <100%> (ø)
src/lin.jl	83.9% <100%> (ø)
src/basis.jl	98.27% <100%> (ø)
src/util.jl	97.87% <100%> (ø)
src/smolyak.jl	87.17% <76.92%> (ø)
src/interp.jl	85.71% <91.3%> (ø)
src/basis_structure.jl	97.61% <96.62%> (ø)

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