fix(libecalc): improve validation of pressures set to zero

[olelod]

Today it is common among ecalc operators to set the rate (and often also pressures) to zero when a consumer should be switched off. At some point in the (not too distant) future, this will change - such that a consumer can be started and stopped in some other way.

The situation today is that it does not really matter what the pressure is when the rate is 0. No calculation will be performed. The code, however, currently treats a zero pressure as missing (None) - and an assert statement fails, due to pressure being required to be defined.

This PR changes the validation of pressures slightly such that the pressure is also required to be positive, but only if the rate is positive. If rate is non-positive, the pressure can be any number.

Type of Work

• Patch: X.Y.Z+1. NEGLIGIBLE visible changes, does not change input or output - OR changes behaviour. Use chore:, refactor: etc
• Minor: X.Y+1.Z. Minor changes, might ADD new input (YAML), or other backwards-compatible changes. Use feat:, fix:
• Major: X+1.Y.Z. Major and most likely BREAKING changes, wo. backwards compatibility, or removing temporary backwards compatibility functionality. Use ! or BREAKING:.

See here (internal): https://github.com/equinor/ecalc-internal/discussions/1044

Have you remembered and considered?

• IF FEAT: I have remembered to update documentation
• IF FIX OR FEAT: I have remembered to update manual changelog (docs/drafts/next.draft.md)
• IF BREAKING: I have remembered to update migration guide (docs/docs/migration_guides/)
• IF BREAKING: I have committed with BREAKING: in footer or ! in header
• I have added tests (if not, comment why)
• I have used conventional commits syntax (if you squash, make sure that conventional commit is used)
• I have included the Github issue nr in the footer!

What is this PR all about?

What else did you consider?

Between the lines?