Domains, operators and degrees

[opeltre]

A couple improvements to be done:

1) Define the empty domain on which every field is zero: this would be useful e.g. for operators like K.delta[0] or K.d[n], and avoid key errors in K[d]

2) There should be a class for operators on the full complex, i.e. K.zeta @ field should call K.zeta[field.degree] @ field, etc.

Most operators are full : K[d].gibbs, K.zeta[d], K.delta[d], etc... but only the +-1 graded ones leave the complex at some point (to zero)

[opeltre]

Empty domains work well with #11 Check creation of null maps with d, delta

[opeltre]

solved by #17