Support 'min' and 'max' time as a replacement for 'center' where needed

[cpiker]

Currently DasDim::getPointVar() only supports

• center
• mean
• media
• mode

however many low-level packets denote data time as the begin point. This is common for waveform captures, particle energy scans, etc. Thus min and max should be considered as appropriate set-point variables for a dimension. This will necessitate updating das3_spice, das3_cdf and related tools including the automatic generation of BIN_LOCATION attributes for output variables.