Improve paper based on JOSS review

[aguspesce]

This is a to-do list for the paper, according to the JOSS reviewer:

• Explain its positioning relative to other software in this space. You don't have to be comprehensive, but please communicate enough about why a researcher would choose this over a representative set of well-known packages.

• The equations can be formatted with \begin{align} or \begin{gather} rather than a sequence of display maths.

• Use other solver in the examples:
  " I see the acknowledgement mentions multigrid, but the van Keken benchmark is configured to use MUMPS. A note on solvers would be useful (even if the intent is that expert users tune using PETSc options)".

• Mention that in a 3D version will be developed:
  "The paper also doesn't mention whether it supports 3D problems and at what scale it has been used. The code isn't entirely helpful in that "future 3d version" appears in comments, while some functions say they are 3d."

openjournals/joss-reviews#3551

[aguspesce]

This issue was solved by #34