Enable reading Semtex field files in FieldConvert

This MR adds a FieldConvert input module to read 2D and 3DH1D field files produced by the spectral element code Semtex. Additionally, a small utility routine InputModule::GuessFormat has been added, which attempts to guess the format of an input file by reading a small amount of data from the header. This allows one to do, e.g.

FieldConvert input.sem.fld output.fld

instead of

FieldConvert input.sem.fld:semfld output.fld

where input.sem.fld is a Semtex field file. This routine supports both Semtex and Nek5000 since they have readily identifiable headers, but not Nektar++ since parsing an XML/HDF5 file and verifying it is for Nektar++ is not a quick task.