[ENH]: Add option to return PDB as a raw string in pyKVFinder.export function

[jvsguerra]

Major changes:

• pyKVFinder.export(fn) now accepts None as the fn input and outputs the PDB-formatted data as a raw string.

Usage:

>>> import pyKVFinder
>>> pdb = os.path.join(os.path.dirname(pyKVFinder.__file__), 'data', 'tests', '1FMO.pdb')
>>> results = pyKVFinder.run_workflow(pdb)
>>> string = results.export(output=None)
>>> print(string)
ATOM     1  H   KAA     0       0.000   0.000   0.000  1.00  0.00
...
ATOM  1000  H   KAA     0       0.000   0.000   0.000  1.00  0.00
>>> string = results.export_all(output=None)
>>> print(string)
ATOM     1  H   KAA     0       0.000   0.000   0.000  1.00  0.00
...
ATOM  1000  H   KAA     0       0.000   0.000   0.000  1.00  0.00

or,

>>> import pyKVFinder
>>> pdb = os.path.join(os.path.dirname(pyKVFinder.__file__), 'data', 'tests', '1FMO.pdb')
>>> atomic = pyKVFinder.read_pdb(pdb)
>>> vertices = pyKVFinder.get_vertices(atomic)
>>> ncav, cavities = pyKVFinder.detect(atomic, vertices)
>>> string = pyKVFinder.export(None, cavities, None, vertices)
>>> print(string)
ATOM     1  H   KAA     0       0.000   0.000   0.000  1.00  0.00
...
ATOM  1000  H   KAA     0       0.000   0.000   0.000  1.00  0.00