Add `AbstractPDMat` constructor

[devmotion]

This PR adds AbstractPDMat(::AbstractMatrix), AbstractPDMat(::Cholesky) and AbstractPDMat(::CholTypeSparse).

The main motivation is that this allows to specialize on the matrix type, which allows downstream packages such as Distributions (e.g. in https://github.com/JuliaStats/Distributions.jl/blob/9f9c5caf6cd99c5205a11ca1c8d6b6471fc9ce29/src/multivariate/mvnormal.jl#L201-L203) to just use AbstractPDMat(A) instead of adding custom dispatches.

Unfortunately, we can't add something like AbstractPDMat(A::UniformScaling) (size unknown) or AbstractPDMat(A::Diagonal{<:Real,<:FillArrays.Fill{<:Real}}) (FillArrays dependency).

Fixes #136 (without lying about the return type).

[codecov-commenter]

Codecov Report

Base: 89.93% // Head: 90.11% // Increases project coverage by +0.18% :tada:

Coverage data is based on head (b4c05b6) compared to base (cc7c5c4). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   89.93%   90.11%   +0.18%     
==========================================
  Files           8        8              
  Lines         437      445       +8     
==========================================
+ Hits          393      401       +8     
  Misses         44       44              

Impacted Files	Coverage Δ
src/generics.jl	86.48% <100.00%> (+0.77%)	:arrow_up:
src/pdiagmat.jl	97.27% <100.00%> (+0.07%)	:arrow_up:
src/pdmat.jl	97.91% <100.00%> (+0.04%)	:arrow_up:
src/pdsparsemat.jl	95.00% <100.00%> (+0.17%)	:arrow_up:

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