Feature/multiple as functions ofs

[robertjensen]

This PR adds the possibility of having more than one as-function-of in an xy-plot.

If graphsettings.xml is left untouched, the old behaviour is retained. To use the new functionality, the syntax in graphsettings.xml is as follows:

<as_function_of>

      <gui>As function of T</gui>
      <column>label</column>
      <reg_match>VTI Temperature</reg_match>
      <xlabel>VTI Temperature</xlabel>

      <b_field>
        <gui>As function of B</gui>
        <column>label</column>
        <reg_match>B-Field</reg_match>
        <xlabel>B-Field</xlabel>
      </b_field>

 </as_function_of>

The first section is the old default syntax, and the following sections (here only a single section named b_field is provided) names the extra as-function-of's.

[KennethNielsen]

@robertjensen I've read though this, but I must admit to being to far out of context to be really useful in a review and I unfortunately don't have the time right now to read back in.

I think it is a good feature to add and as best I can, I saw no immediate problems with the code, but considering how far out of context I am, regard my review on both code and architecture, as being done by someone not really qualified.