Docs: Add note to use psi4.core.clean after calculations

[KB1RD]

I am new to Psi4 and quantum chemistry in general. The PsiAPI tutorial is helpful in getting started, but doesn't mention use of psi4.core.clean(). I can't find any mention of this function in the docs, except here.

A newbie like me may try to run two calculations in series. In my case, I got:

Traceback (most recent call last):
  File "test.py", line 47, in <module>
    h2oi = energyFromGeometryFile('geometries/01.xyz', method="ccsd")
  File "test.py", line 27, in energyFromGeometryFile
    return calculateEnergy(text, method, basis)
  File "test.py", line 12, in calculateEnergy
    e = psi4.energy(method + '/' + basis)
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/driver.py", line 525, in energy
    wfn = procedures['energy'][lowername](lowername, molecule=molecule, **kwargs)
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/procrouting/proc.py", line 871, in select_ccsd
    return func(name, **kwargs)
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/procrouting/proc.py", line 2894, in run_ccenergy
    ref_wfn = scf_helper(name, **kwargs)  # C1 certified
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/procrouting/proc.py", line 1887, in scf_helper
    e_scf = scf_wfn.compute_energy()
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/procrouting/scf_proc/scf_iterator.py", line 82, in scf_compute_energy
    self.initialize()
  File "/home/ubuntu/build/build/stage/lib/psi4/driver/procrouting/scf_proc/scf_iterator.py", line 197, in scf_initialize
    self.form_H()
RuntimeError: 
Fatal Error: PSIO_ERROR: 18 (Incorrect block end address)

Practical advice: This error is probably disk related. First, check that your scratch isn’t full.
Second, make sure that you’re setting scratch ("psi4 -s" or "export PSI_SCRATCH") rather than letting it 
default to /tmp. Third, run the job again to see if the error is reproducible.
If it persists, finally, file an issue.

Error occurred in file: /home/ubuntu/build/psi4/src/psi4/libpsio/error.cc on line: 142
The most recent 5 function calls were:

psi::PsiException::PsiException(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, char const*, int)
psi::PSIO::write(unsigned long, char const*, char*, unsigned long, psi::psio_address, psi::psio_address*)
psi::PSIO::write_entry(unsigned long, char const*, char*, unsigned long)
psi::Matrix::save(psi::PSIO*, unsigned long, psi::Matrix::SaveType)

Turns out, this is caused by not running psi4.core.clean() between these calculations. To find this function, I took a lucky guess that Psi4 had some kind of cleanup function since independent calculations succeeded. Instead, this should probably be in: https://psicode.org/psi4manual/master/psiapi.html

I am fully willing to submit a PR with this documentation, but I don't have much experience with Psi4/QC knowledge in general, so I'm worried I might leave something out.