Improve and uniformize naming

[maxtremblay]

I propose the following nomenclature to uniformize the names within CubeCL.

Nomenclature

Indexing

• Use index for linear (contiguous) buffers, slices and arrays.
• Use coordinate for the cartesian coordinate of a tensor element.
• Use pos or position for cube related objected (CubePos, UnitPos)
• Use axis instead of dim for tensor.

Objects in containers

• Use element for a value inside a tensor or indexing a line.
• Use line or something_line when indexing returns a Line<T>.
• A container is lined if it stored Line<T> instead of T. (Currently both lined and vectorized are used).

Example

let element_line = tensor[index];
let element = element_line[0];

Length, size and shape

• Use len (or length) for linear memory to match rust convention and lines.
• Use size for the total number of elements in a tensor or in a line. Thus we always use line_size, never vectorization or vectorization_factor or other similar variations.
• Use shape for the length of a tensor's axis. The shape of a tensor is a list (vec, slice or tuple) of the shape of each axis.
• Use rank for the number of axes in a tensor.
• Prefer something_count instead of num_something. (ex: CubeCount, unit_count)

Tensors

• An axis is contiguous if its stride is 1.
• The lines of a tensor are parallel to an axis if either it is the only contiguous axis or if it is contiguous and it has a shape greater than 1. Otherwise, the axis is perpendicular.
• A tensor is in increasing_order if stride(axis1) < stride(axis2) whenever axis1 < axis2. This is also named column_major_order when the tensor is a matrix (rank == 2).
• A tensor is in decreasing_order if stride(axis1) > stride(axis2) whenever axis1 < axis2. This is also named row_major_order when the tensor is a matrix (rank == 2).
• A tensor is in non_monotonic_order if it is neither in increasing_order nor decreasing_order.
• A stride is all the elements obtained when fixing the coordinates of all axes but one.

Meta

• Prefer postfix names such as ReduceError and ReduceStrategy.
• Avoid abbreviation except if they are really common and widely used.
• Create a conventions.md file in the project root to help users and contributors follow the naming convention (and possible other conventions in the future).