Move utility functions to +utils sub package

[ilent2]

It would be nice to move the utility functions to a +utils sub package. We could then also drop the backwards compatibility requirement on the utility functions (say they are for internal use and users should use them at their own risk).

Functions I think should be included in the +utils directory, there may be others...

• angulargrid.m
• spharm.m
• vsh.m
• iseven.m
• rtp2xyz.m
• vswf.m
• isodd.m
• rtpv2xyzv.m
• wigner_rotation_matrix.m
• sbesselh1.m
• xyz2rtp.m
• laguerre.m
• sbesselh2.m
• xyzv2rtpv.m
• legendrerow.m
• sbesselh.m
• z_rotation_matrix.m
• sbesselj.m

[AStilg]

translate_z is in the same category.

[ilent2]

Done