Refine tensor transform interface

[dlfivefifty]

This refines the tensor transform interface. In particular:

1. grid(P) and grid(P,n::Int) return just a vector of points
2. grid(P, (n,)), grid(P, (m,n)) etc. return a tuple of vectors of points in each dimension.
3. Similar for plan_grid_transform.

Thus one can do a 1D transform that gets out at least 5 coefficients via eg:

x = grid(P, 5)
P = plan_transform(P, 5)
P * f.(x)

or a 2D transform

x,y = grid(P, (5,6))
P = plan_transform(P, (5,6))
P * f.(x, y')

Note the "1D" and "2D" don't necessarily mean dimension of the geometry, eg, when combined with MultivariateOrthogonalPolynomials.jl the interface of a tensor product of disks would be:

Z = Zernike()
x,y = grid(Z, (Block(5),Block(6)))
P = plan_transform(Z, (Block(5),Block(6)))
P * f.(x, y')

@TSGut this might be of use for you.

[dlfivefifty]

Now only overload plan_transform and grid: plan_grid_transform won't be used anymore.

A consequence is that the default fallback transform of using Vandermonde+LeastSquares creates the grid twice. Whooptydoo.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (5bcf055) 91.59% compared to head (88f606e) 91.69%.
Report is 8 commits behind head on master.

Files	Patch %	Lines
ext/ContinuumArraysMakieExt.jl	0.00%	3 Missing ⚠️
src/maps.jl	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #170      +/-   ##
==========================================
+ Coverage   91.59%   91.69%   +0.10%     
==========================================
  Files          11       11              
  Lines         761      783      +22     
==========================================
+ Hits          697      718      +21     
- Misses         64       65       +1     

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