ReplaceFemSpace.spaces and FemField.fields with more specific attributes

[yguclu]

Let Vh be a FemSpace. Currently Vh.spaces may return different types of spaces, which makes the code not always easy to read, and hampers the single/multi-patch compatibility. Instead of this attribute, we should provide more specific helper attributes:

• Vh.patch_spaces[i]:
  if Vh is a multi-patch space (ie ProductFemSpace), return FEM space on patch i
  if Vh is single-patch (anything else) return itself (and raise Error if i>0 ?)

• Vh.component_spaces[i]:
  if Vh is a multi-patch space (ie ProductFemSpace), raise error
  if Vh is single-patch, vector-valued (ie VectorFemSpace) return space of component i
  if Vh is single-patch, scalar-valued (ie TensorFemSpace) return itself (or error? only if i>0?)

• Vh.axis_spaces[i]:
  if Vh is single-patch, scalar-valued tensor-product (ie TensorFemSpace), return space for axis i
  else raise Error ?

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A similar problem occurs for a multipatch/vector-valued FemField uh (uh.fields may correspond to patch or component fields). Instead of this attribute, we should provide more specific helper attributes:

• uh.patch_fields[i]:
  if uh is a multi-patch field returns FEM field on patch i
  else returns itself (raise error if i>0 ?)

• Vh.component_fields[i]:
  mimic behavior of Vh.component_spaces?