fix bug in __iter__
big fixes
io.nc, when reading:
- remove singleton axes
- convert unicode to str

indexing:
- chained call was buggy: just made it simpler to fix it (but less efficient, come back to it later?)

axes:
- raise_error fixed for string, by chaning ValueError to IndexError ==>  reindexing now also works with string axes
- isnumeric method: now actually check by doing an addition instead of axis type
- check Axis values at init : not a scalar
- Axes: check that every element is an Axis in __init__

missingvalues: minval parameter now working