core and tensor - add new "equiv" method to BandMat (and use for a slightly more efficient plus)

Ignore-this: 74a28a1128024e870348e24eb0a63a4d

darcs-hash:20130923172411-3a4db-07c8a97b6e23340a5d4ef15d8958eed875ea3293
(testhelp - slightly more explicit (and better formatted) assert functions)

Ignore-this: d25e93bb6c4fb763915babf96f7cc73a

darcs-hash:20130923225244-3a4db-8a80bf4fb21aeee2041ea3555d8be09bb4ecadc9
core and tensor (and others) - convert some functions in tensor to infix operators in core

Ignore-this: 62c7b026d0150058be9ded0768f08352

darcs-hash:20130924012201-3a4db-e2ec10043134e735582b3d044577680393f84bad