Issue240 better ref com msg

[Didou09]

Main changes:

More informative warning msg when testing dref_vector in get_ref_com(), thanks to dref_vector_name

Issues:

Fixes, in devel, issue #240 240

Example:

In [4]: dax = coll.plot_as_profile2d('cprof_1djpar', dres=0.03)
/data/home/dvezinet/projects/datastock/datastock/_class1_uniformize.py:202: UserWarning: Multiple possible vectors found:
['cprof_t', 'cprof_m1d_bs1_ap']
	- key0: cprof_1djpar
	- key: None
	- ref: None
	- hasref: None
	- refok: ('cprof_nt', 'cprof_m1d_bs1_nbs')
Provided:
	- dim = None
	- quant = None
	- name = None
	- units = None
From 'dref_vectorZ'