Prevent CHEBMATRICES from having non-scalar block entries.

Closes #806.

This entailed some faffing around in LINOP/EIGS() and CHEBGUI, which were guilty of breaking this rule.
Some improvements to eigenmode plotting.
Merge branch 'development' of ssh://github.com/chebfun/chebfun into fix-chebmatrix-ctor
Using linewidth=2 for plotting eigenmodes of systems.
Making LINOP EIG tests consistent with new CHEBMATRIX conventions.
Updating CHEBOP EIG test to work with new CHEBMATRIX convention.