Add factory to create electronic solver instances

[awvwgk]

Closes #68

cc @cbannwarth

[codecov]

Codecov Report

Merging #69 (f740a38) into main (dcfb318) will decrease coverage by 0.53%.
The diff coverage is 36.32%.

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   73.12%   72.58%   -0.54%     
==========================================
  Files         119      123       +4     
  Lines       15268    15489     +221     
  Branches     4505     4607     +102     
==========================================
+ Hits        11165    11243      +78     
- Misses       2084     2182      +98     
- Partials     2019     2064      +45     

Impacted Files	Coverage Δ
app/cli.f90	22.25% <0.00%> (-0.65%)	⬇️
app/driver_run.f90	24.24% <0.00%> (-0.19%)	⬇️
src/tblite/lapack/potrf.f90	28.57% <28.57%> (ø)
src/tblite/lapack/sygst.f90	30.00% <30.00%> (ø)
src/tblite/lapack/sygvr.f90	34.23% <34.23%> (ø)
src/tblite/blas/level3.f90	15.33% <35.29%> (+5.85%)	⬆️
src/tblite/context/type.f90	62.50% <60.00%> (-1.14%)	⬇️
src/tblite/lapack/solver.f90	64.70% <64.70%> (ø)
src/tblite/lapack/sygvd.f90	32.35% <66.66%> (-3.14%)	⬇️
src/tblite/xtb/singlepoint.f90	78.36% <66.66%> (+0.12%)	⬆️
... and 1 more

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