Fix #117

[cortner]

• fix #117
• started implementing tests of docstrings
• improve type signatures to allow auto-conversion of cell vectors and PBC

The last point is the main change: introduced AUTOBOX and AUTOCELL union types, added those to various function type signatures. These unions describe which input types for cell vectors / bounding box and pbcs can be converted into the required format via _auto_cell_vectors and _auto_pbc (cf. cell.jl). E.g. internally the cell vectors are always stured as NTuple{D, SVector{D, T}} but it can be more convenient to construct them as

[[ a, 0, 0 ], [0, b, 0], [0, c, d] ] * u"nm"

and _auto_cell_vectors will then convert.

[cortner]

@lxvm -- let me know if this addresses your issue.

[cortner]

@rkurchin @mfherbst @tjjarvinen -- I need one of your reviews please to merge.

[cortner]

thank you