Added deprecation warning for Matrix \\ Matrix

[mahrud]

This PR is in preparation for a PR in the next release which will change g \\ f to call quotient'(f, g) instead of being a synonym for f // g = quotient(f, g), and which will add a new algorithm that works for arbitrary modules rather than only free modules, as well as maps of coherent sheaves. This change was suggested in #1448 during work on PrimaryDecompositions with @mikestillman, and recently came up again during work on Varieties with @ggsmith and others.

Here is the warning:

i1 : M = ZZ^3;

i2 : id_M \\ id_M
 -- Note: 'm \\ n' is deprecated; use 'n // m' instead.

o2 = | 1 0 0 |
     | 0 1 0 |
     | 0 0 1 |

              3       3
o2 : Matrix ZZ  <-- ZZ

Note that the current algorithm for quotient' is even more limited than quotient, because dual doesn't work for maps between non-free modules:

i1 : M = coker(5 * id_(ZZ^2));

i2 : f = map(M, M, {{1,2},{3,4}});

o2 : Matrix M <-- M

i3 : g = map(M, M, {{2,3},{4,0}});

o3 : Matrix M <-- M

i4 : quotient(f * g, f)

o4 = | 2  -2 |
     | -1 0  |

o4 : Matrix M <-- M

i5 : quotient'(f * g, g)
stdio:5:1:(3): error: expected maps between free modules

Lack of good notation and hard to find documentation has lead to people resorting to code such as this:

M2/M2/Macaulay2/packages/BettiCharacters.m2

Lines 569 to 571 in 615b0e6

	-- it is necessary to transpose because we need a left factorization
	-- but M2's command // always produces a right factorization
	transpose(transpose(C.dd_(i+1))\\transpose(sub(C.dd_(i+1),gInv)*g0))

Other change:

• added deprecation warning for Matrix \ Matrix
• removed Matrix \ Matrix from documentation
• removed Matrix \ Matrix from BettiCharacters
• added List*Thing, replaced List/Thing
• fixed an incorrect test in Quasidegrees