DOC: docstring for PARRECArrayProxy
TST: make truncated PAR/REC and test

Test that truncated PAR/REC file raises error or warning according to
`permit_truncated` flag.
RF+TST: add permit_truncated to header; fix copy

Add `permit_truncated` to header attributes. Fix `copy` method to use
value of `permit_truncated` when making a copy. Test.
RF+TST: standard file loading methods for PARREC

Add methods with optional args `permit_truncated` and `scaling`:

* from_file_map
* from_filename
* load
NF: add keyword-only decorator with tests

Add decorator to make keyword arguments keyword-only for Python 2.
RF: refactor PARREC file meths for keyword only

Use and test keyword-onliness of permit_truncated, scaling argument to
PARRECImage file loading methods.
Merge pull request #264 from matthew-brett/parrec-fix-refactor

MRG: some refactoring of PARREC API

Add fake truncated file for testing, and test.
Add permit_truncated as header attribute.
Fix header copy to use permit_truncated.
Add file loading API methods with PARREC-specific parameters.
Make PARREC-specific parameters keyword-only.