Add LOG2-VALUE to the list of allowed protein quantification data types

[rmadupuri]

Addresses user question: https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/cbioportal/xWOLy5mhk44/zB9CqzsdAAAJ

[rmadupuri]

@jjgao the other profiles with CONTINUOUS data type are Methylation and Expression. Do we have to allow LOG2-VALUE for these as well? In that case, we will have to update the File formats page as well. See https://github.com/cBioPortal/cbioportal/blob/master/docs/File-Formats.md#methylation-data and https://github.com/cBioPortal/cbioportal/blob/master/docs/File-Formats.md#expression-data. Thanks!