Miscellaneous small fixes

[TomMaullin]

This PR addresses the following miscellanoues small issues:

• issue #16 : There is now a check performed on the rank of X'X
• issue #17 : The volume blmm_vox_resms is now optional.
• issue #19 : PLS has been renamed to PeLS throughout the code.
• issue #20 : All inv's have been replaced by either solve or pinv depending on context.