Incorrect delegation from `String` to `ViewString`

[fingolfin]

The GAP library has a bewildering number of functions for printing/viewing/displaying objects to the terminal or into a string. There is a strict hierarchy how methods for the various involved operations may delegate, described in the GAP reference manual.

The following method violates this:

homalg_project/MatricesForHomalg/gap/HomalgMatrix.gi

Lines 3914 to 3918 in 698bcb6

	InstallMethod( String,
	"for homalg matrices",
	[ IsHomalgMatrix ],
	ViewString );

To fix this, just install the ViewString method for IsHomalgMatrix as a method for String instead and then delegate "the other way around".

[mohamed-barakat]

If I do the proposed change the test suite fails for the following reason: ViewString is an operation that alters its output depending on how much the matrix knows about itself, contrary to String which is an attribute.