nektar

FilterFieldConvert

This MR will introduce a new filter, which will be able to run modules from FieldConvert on a checkpoint. This filter replaces FilterSampler, and therefore the new functionalities will extend to other filters derived from it (AverageFields, ReynoldsStresses). An example of the setup for that would be:
``` xml
<FILTER TYPE="FieldConvert">
    <PARAM NAME="OutputFile"> filename.vtu </PARAM>
    <PARAM NAME="OutputFrequency"> 100 </PARAM>
    <PARAM NAME="Modules"> vorticity homplane:planeid=4 </PARAM>
</FILTER>
```
It is unlikely that all options and modules will be supported. However, the main idea is to use this with modules that result in outputs with reduced size (e.g. isocontour, meanmode for 3DH1D). This way we can obtain these outputs more frequently than would be practical by storing the full checkpoint and running FieldConvert later.

I don't know how far I am from finishing this (some cases already work, but I still need to test it further). Anyway, it would be nice to have some feedback on it, especially regarding the compilation and general structure of the code, since I had to create a new FieldConvert library and move some files around.

See merge request !589