Ensure `Base.one` and `Base.zero` are implemented where possible

[tpgillam]

zero should be definable for all multivector types. one can be defined for even multivector types, where it should correspond to the scalar 1 -- but it should remain undefined for odd multivectors.

Currently it seems to be defined for instances, but often not for types.

[tpgillam]

This is needed to be able to construct an array of zeros or ones, e.g.

using SimpleGA
zeros(GA30.Odd{Float64}, 4)

Currently we get the error:

  MethodError: no method matching SimpleGA.GA30.Odd{Float64}(::Int64)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     Closest candidates are:
    (::Type{SimpleGA.GA30.Odd{T}} where T<:Real)(::Any, ::Any, ::Any, ::Any)
     @ SimpleGA ~/.julia/packages/SimpleGA/8UwZx/src/core30.jl:15
    (::Type{T})(::T) where T<:Number                                                                                                                                                                                                                                                             @ Core boot.jl:792
    (::Type{T})(::Base.TwicePrecision) where T<:Number
     @ Base twiceprecision.jl:265
    ...
                                                                                                                                                                                                                                                                                              Stacktrace:                                                                                                                                                                                                                                                                                   [1] convert(::Type{SimpleGA.GA30.Odd{Float64}}, x::Int64)
      @ Base ./number.jl:7
    [2] zero(::Type{SimpleGA.GA30.Odd{Float64}})
      @ Base ./number.jl:309                                                                                                                                                                                                                                                                    [3] zeros(::Type{SimpleGA.GA30.Odd{Float64}}, dims::Tuple{Int64})
      @ Base ./array.jl:637
    [4] zeros(::Type{SimpleGA.GA30.Odd{Float64}}, dims::Int64)
      @ Base ./array.jl:632