began to implement pytest tests.

- converted tests for read
- converted tests for seek
- converted tests for write
refactored test fixtures

this simplifies the fixture setup some. A lot less repetition now.
converted a few more tests
renamed read test file to test_r

now symmetric with test_w
added preparation for rw mode tests
added tests for rw mode and fixed related bugs

There are some peculiarities with how to set the file format when the
file is in rw mode. Now, file format is optional for rw mode.
added mono tests
add another test file.

This should have been part of the last commit.
Remove obsolete imports
Rename duplicate test functions
Some (mainly automatic) PEP 8 formatting changes
fix for reading RAW files

also added regression test for this
changed test files to contain unique data

thus testing for data permutations
Revert ValueError to TypeError for missing args

Sorry for the mess.
Merge pull request #48 from bastibe/tests

Implement Unit Tests using pytest.