Add support for Reading `CMTSOLUTION` and `FORCESOLUTION`

## Task description

Adding extra readers to adapt for classic formats. Specfem3d now allows for both formats together. So, the user should be able to set two separate files.

- [ ] Add CMTSOLUTION reader
- [ ] Add FORCESOLUTION reader
- [ ] Add io tests for reading the files

## Acceptance criteria
- [ ] tests pass
