Potential issues in M7 for Single Precision

[plesager]

Try to identify and fix potential issues when running in SP.

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[plesager]

An idea is to devise a the test independent from the precision, for example by using a fixed lower value of 1e-30 (or just 2e-16 so we are closer to what EPS in DP is currently using)

[noije]

Also, in module/mo_ham_m7.F90, this line:

zrpav=zeps+0.5_dp*(pm6rp(jl,jk,jm1)+pm6rp(jl,jk,jm2))

does not seem to be safe. With pm6rp in cm, the addition of zeps in SP boils down to a shift by zeps=1.19e-7 cm or ~1 nm.

[plesager]

The ERF function is default in Fortran 2003 and above. We could even use the MKL library with Intel, but that can be left for future improvement.
Using erf here removes all the tests on eps and zmin introduced in that subroutine.

[noije]

In general, with EPSILON()=1.19e-7 in SP, we cannot guarantee any longer that it is used correctly throughout the HAM-M7 code, and we need to check this carefully.