Prevent computations in float16 in scalar and elemwise

Catch exception if open() fails as well.

Also:
- add tests for "inv" op
- do not test inplace if the input is int and the output is float
- remove a couple of redundant dicts

Otherwise, the expected values could be computed in float16,
which would not be precise enough, and which would not be supported
as a dtype by Theano.

Since many elemwise ops do not call the scalar op's impl() method,
we needed to also change the perform method when numpy built-in
ufuncs are used.

Ops that are defined with upcast_to_float should upcast to
float32 or float64 minimally.

Add tests for the cases where the inputs were int8, which is when
float16 values appeared.

Drop support for python 2.4.

missing c_set_tensor4d in GpuDnnConvBase

Fixes #633

Mixed

This help gpuarray tests to run on device that don't support float64.

fix gh-2224