Improved Markdown formatting of INSTALL.md file.
Improved robustness of options.mk.sample file. Now strips whitespace out of USE_CPP11 value and issues an error if USE_CPP11 undefined or set to any value besides yes or no.
Added a check for negative prime level.
Partial work on adding features to Spectrum class.
Expanded features of Spectrum class and added ability to construct from tensors containing singular values.
Merge branch 'spectrum'
Commented out printing in spectrum_test.cc
Made IQIndexVal and IndexVal no longer subclasses of IQIndex and Index. This design was leading to confusing behavior when trying to construct an ITensor from one or more IQIndexVal (which are often used by the SiteSet classes). Added some related unit tests.