Keep existing stations downloading new coincidences

When downloading new coincidences from the esd
Only stations in the newly downloaded data were
included in the cluster that was written to the file.
Now stations in the existing cluster object are also included.
Fixes #35.
Test fixed

Station 501 has been moved, new location unknown for now
Code cleanup, some PEP8 fixes
Removed error in regression algorithm

Removed a stupid bug in regression.
Also corrected the fit part.
Adapted test script for a not exactly zero result (no big deal but
otherwise confusing failure messages show up).
Added class SphereAlgorithm

Uses the hypotheses that particles travel with the speed of light.
Fixed typos
Fix no _v_attrs bug in download coincidences

When the coincidences group is first created in the HDF5 file, it is
not returned by the create method. The result is that None is returned
instead.
PEP8 fixes
Update SphereAlgorithm

Improved variable names.
Fix consistency (sqrt vs ** 0.5).
Update docstring.
Improved indentation for clarity.
Merge pull request #43 from HiSPARC/refactor_reconstructions

Fixed regression and fit algorithms and add sphere algorithm.
Also perform PEP8 fixes. Fix bug in download coincidences.