GitLab

* enable atom subset in PDBReporter

* adds optional atomSubset argument to PDBReporter
* adds functions in PDBReporter which create a new topology
  and positions with chosen subset of atoms
* PDBReporter write PDB files with the subset topology and positions.

* refactor PDBReporter

* make _createTopologySubset a method that is called only the first time it is needed
* more efficient creation of subset positions
* check that atomSubset is ordered

* refactoring PDBReporter and add PDBx

* move checks on atomSubset to _createSubsetTopology
* copy periodic box vectors from topology to subsetTopology
* add atomSubset to PDBx reporter

* add bond subset to createSubsetTopology

* formatting changes to pdbreporter

* loop over atoms cleaned up
* put file opening inside parameter loop in
  TestPdbReporter::testinvalidSubsets to try and fix failing tests on windows

* add thorough tests to TestPdbReporter

* spelling changes
* Add tests for atom positions, elements, names etc
* attempt at fixing PyPy and Windows failing test cases

* fix spellings

* close output file before raising exceptions

* closes output files before raising exception in pdbreporter
createSubsetTopology
* changes assertVecAlmostEqual from a method to function to avoid
  repeating