Mapping of a coordinate to another. It supports analytic and
gridpointbased mapping in subclasses. Here in this root
CoordMapping class only the invariant unity mapping (or no mapping)
is defined.

Constructor. One or more arguments can be needed in subclasses

Maps data point(s)
ARGUMENTS
 x,y,z,... (one or more Numeric or NArray) : data points.
Mapping is made of [x,y,z,..] (if Numeric) or
[x[0],y[0],z[0],..], [x[1],y[1],z[1],..], ..(if NArray).
Thus, the number of arguments must be equal to the rank of
the mapping. Also, their lengths must agree with each other.
RETURN VALUE
 Array of p,q,r,... (Numeric or NArray) : mapping result

Same as map but for a regular grid.
ARGUMENTS
 x,y,z,... (one or more 1D NArray) : coordinate values of
a regular grid [x_i, y_j, z_k,..]. The shape of the grid is thus
[x.length, y.length, z.length,..]. This method needs no redefinition,
since it calls map inside.

Inversely maps data point(s).
ARGUMENTS
 p,q,r,... (one or more Numeric or NArray) : data points.
Inverse mapping is made of [p,q,r,..] (if Numeric) or
[p[0],q[0],r[0],..], [p[1],q[1],r[1],..], ..(if NArray).
Thus, the number of arguments must be equal to the rank of
the mapping. Also, if NArray, their lengths must agree with each other.
RETURN VALUE
 Array of x,y,z,... (Numeric or NArray) : inverse mapping result

Returns the inverse mapping.
RETURN VALUE

Whether the inversion is rigorous (analytical)
RETURN VALUE
Linear coordinate mapping expressed as offset + factor*x,
where offset is a vector (NVect) and factor is a matrix (NMatrix).
Methods listed below are only those newly defined or those whose
arguments are changed.

Constructor. If one of offset and factor is not
specified (nil), a zero vector / a unit matrix is used (at least
one of them must be given).
ARGUMENTS
 offset (NVector or nil) : the offset. Its length represents the rank
of mapping. (if nil a zero vector is assumed)
 factor (NMatrix or nil) : the factor. For consistency,
( offset.length == factor.shape[0] == factor.shape[1] ) is required.

Returns the internally stored offset.

Returns the internally stored factor.