[#138] Fixed indexing in operator _D1_Deg0_2d()

[Didou09]

Main changes:

Following the changes to bsplines indexing implemented in 0.0.28, one of the operator also had to be updated:

• _utils_bsplines_operators._D0N1_Deg0(): updated for new indexing

• plot_mesh(key=key_bs) now works

• plot_bsplines_operator(operator='D1N2') operational

Issues:

Fixes, in devel:

• issue add_bsplines_operator() broken for m1_bs0 operator='D1N2' #138
• issue plot_mesh(key=key_bs) should work #137

Examples:

import tofu as tf

# test 
test = tf.tests.tests07_inversions.test_01_isotropic.Test01_Inversions()
test.setup_class()
test.setup_method()

# plot bsplines operators
dax = test.coll.plot_bsplines_operator('m1_bs0', centered=False)
dax = test.coll.plot_bsplines_operator('m1_bs1')
dax = test.coll.plot_bsplines_operator('m1_bs2')