Apply a few fixes and improvements from PDMats.jl

[devmotion]

This PR adapts a few improvements from the latest PDMats releases and fixes a StackOverflowError with quad and invquad that show up due to an updated dispatch hierarchy in PDMats (it's required to define the matrix versions as well: the previous default was wrong and implemented with inplace operations which broke StaticArrays support).

[codecov[bot]]

Codecov Report

Merging #35 (729741b) into master (fcf559d) will decrease coverage by 1.61%. Report is 4 commits behind head on master. The diff coverage is 61.11%.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   74.13%   72.53%   -1.61%     
==========================================
  Files           4        4              
  Lines         116      142      +26     
==========================================
+ Hits           86      103      +17     
- Misses         30       39       +9     

Files	Coverage Δ
src/PDMatsExtras.jl	100.00% <ø> (ø)
src/utils.jl	88.88% <0.00%> (-11.12%)	:arrow_down:
src/psd_mat.jl	57.83% <62.85%> (+4.38%)	:arrow_up: