UPDATED Pulling to load metadata, better filehandling

Constructors now simply load **metadata arguments to metadata instance
variable. This is done automatically when using Pulling.fromFile().

fromFile() can be used the following ways:
[assume test.str and test.fret exist]

Pulling.fromFile('test.str') # pulling data only + metadata
Pulling.fromFile('test.str','test.fret') # pulling/fret data
Pulling.fromFile('test') # pull/fret data
DELETED loadPull(): obsolete