update_spahm-b

[YAY-C]

There are a few changes I would like to PR:

• optional argument to use the same basis (CC.bas) for all atom-pairs
• changes to generate representations for a single element-type in a structure
• ecp argument for CLI parser

then I also have various some implementations for a better atom-pair bagging for global spahm-b, but these are not totally worked out yet

Is it ok if I included these changes in the same PR or do you want me to split

[briling]

I think ecp should be together with other ecp changes. The other two are fine together if they're in separate commits

[YAY-C]

Ecp related changes are now at #48