BUG: Fix indexing 0-sized arrays using empty fancy-indexes

Due to subspace creation for fancy indexing selecting the 0s
element on every fancy dimension, this failed if the dimension
was empty. Propagating check_index to disable the unnecessary
check in this case.
ENH: Allow newaxis in combination with fancy indexing.

This commit allows the use of None when also using fancy indexing.
To achieve this mit->consec is used to note the denote the
transposition axis (which means that if its consecutive but at the
start, mit->consec==0, and no transposition is necessary).
BUG: Check for maximum dimension for fancy indexing.
BUG: Do not ignore subspace unless it is true 0-d

This is  more important when allowing newaxis, but already before
lead to successful indexing when it should not be suspected.
Also some maintenance.
MNT: Tiny cleanup for mapping.c and comments
TST: Add fancy indexing test for tuple indexing

This is a first version of fancy indexing tests by mimicing the
behavoir in python. Note that error messages are not tested.
DOC: Advanced indexing using np.newaxes

Mention in release notes and remove documentation which may
give the idea it cannot be used with advanced indexing.
MAINT: Remove unnecessary and wrong check for mapiterswap

With the slight modification of the MapIter API to allow newaxis
in fancy indexes, the MapIterObject consec field is already the
axis to transpose to, and with a leading newaxis, even if
iteraxes[0] is 0, consec may not be 0.
Merge pull request #2701 from seberg/indexing

Fancy Indexing enhancements and bug fixes