Improved file reading using vtk rectilinear mesh

[supriyoumich]

These commits are for external file reading with vtk rectilinear mesh. Currently only unstructured mesh can be read through the input file. However, many problems do not require unstructured mesh. Therefore, rectilinear mesh can reduce the file size and improve the memory usage while reading the files. The default option for file reading is set as unstructured mesh. If you want to read rectilinear file instead, include the following line just below "set Load grain structure" in your input parameter file:
set Load as unstructured grid = false

I compiled the code and it worked for me, hopefully this can be merged with the master.

[david-montiel-t]

Could you instead do the pull request to the development branch so that it goes through the CI check and formatting tests?

Also, I think it would make sense for the flag to be something like

set Input vtk file dataset type = UNSTRUCTURED
or
set Input vtk file dataset type = RECTILINEAR

with the default set to UNSTRUCTURED

This is more in line with other options in the parameters file.

[supriyoumich]

I will be doing a new pull request for this