"""Return the value for a key after parsing ~/.theano.cfg and
the THEANO_FLAGS environment variable.
We parse in that order the value to have:
1)the pair 'section.option':value in default_
2)The ~/.theano.cfg file
3)The value value provided in the get*() fct.
The last value found is the value returned.
The THEANO_FLAGS environement variable should be a list of comma-separated [section.]option[=value] entries. If the section part is omited, their should be only one section with that contain the gived option.