set GP prior for non-mechanistic infection model

[avallecam]

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Aims to Fix #96

• What is the current behavior? (You can also link to an open issue here)

In {EpiNow2} version 1.7.1 the default GP prior is now sd = 0.01. The non-mechanistic infection model used in the estimate infections howto entry was built assuming a GP prior of sd = 0.05 (version 1.6.1). Given that the new default GP prior is more appropriate for the renewal equation model (rt!=NULL) than the non-mechanistic infection model (rt = NULL), the non-mechanistic callings in the corresponding howto entry now specify explicitly sd = 0.05 for the GP prior. For further discussions on this regard, refer to previous and latest issues linked to: epiforecasts/EpiNow2#829

• What is the new behavior (if this is a feature change)?

The website will keep rendering appropriate infection estimates when upgrading EpiNow2 CRAN version 1.7.1.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

[avallecam]

issue in GH actions to solve in #100

[avallecam]

now runnable checks are passing, so will merge 🚀 thanks!