Indexing rework for 0.2

[brainandforce]

Because we currently disallow bare integer indices for AbstractCliffordNumber instances, we could rework the indexing scheme to allow for integer indices if the inputs to getindex are passed through to BitIndex:

getindex(x::AbstractCliffordNumber, i::Integer...) = getindex(x, BitIndex(x, i...))

This is a relatively naive definition; we may want to perform checking so that a BitIndex of a grade constrained to be zero by the type of x automatically returns zero without performing any construction.

[brainandforce]

This actually may not be a breaking change, and can go into the 0.1 series.

[brainandforce]

Correction: this might be a breaking change, and not the good kind.

There is a default definition of Base.getindex(::Number) Base.getindex(::Number, ::Int) that returns the Number argument if the integer argument is equal to 1 (or if no argument is supplied). Defining this method may break some code that relies on the fact that AbstractCliffordNumber behaves as a scalar.