Make sys.F90 an optional requirement

[Nick Papior]

It may not always be advantageous to have a sys.F90 a requirement in terms of the required code interfaces.

What I suggest would be the LAPACK like solution:

suffix subroutines with an `info` integer.

The return values:
- negative numbers correspond to an invalid argument corresponding to the position: abs(info)
- positive numbers are internal errors for one or the other reasons.

Possibly we could have a netcdf-like API:

call gridxc_cellxc(..., info)
call gridxc_err(info, [msg[, ?]])

This would make hosting application able to do some responses at least.

The report counters (memory and timers) should also be optional, possibly using procedural pointers in the init-call?

Well, thoughts and rants, please comment!

[Nick Papior (nickpapior)]

Additionaly the sys.F90 does not match what we really need.

There is a mix of

alloc_meomry_report
alloc_error_event

which should be fixed in any case.