CanSas HDF reader will not read all valid CanSas HDF files

[pkienzle]

The current canSAS reader is interpreting fields by name even when the name of the field is not specified in the canSAS spec:

​http://download.nexusformat.org/doc/html/classes/applications/NXcanSAS.html

For example, the mask information is being read from the "Mask" field, but the spec says that the mask field name given by the required @mask attribute of the SasData group. It may happen to work since "Mask" is a likely field name for the mask data, but "mask" is equally likely and HDF is case sensitive.

It may be that the reduction software is not writing the required attribute names, but the reader should at least check prior to looking for the default names such as "mask" or "Mask".