reparameterise PyArray

[cjdoris]

Changes the parameters of PyArray from PyArray{T,N,M,L,R} to PyArray{T,N,F}:

• F is a tuple of symbols, encoding various flags.
• The old M parameter becomes the :mutable flag.
• The old L parameter becomes the :linear flag.
• Also adds a :contiguous flag and adds that for even faster access.
• The old R parameter is now implied by T.

The last point means that we no longer allow python objects to be converted to arbitrary eltypes with pyconvert. We have an explicit mapping of T to R. For now, T must be a basic scalar type which equals R or T=Py.