add Gamma distribution for entoAvailability

[acavelan]

Add the possibility to sample the host availability from on the following distributions:

• const (default)
• lognormal with CV or variance
• gamma with CV or variance

It can be specified via the <availability ... /> parameter in the entomology section of the xml, for each species.

Some examples include:

<availability /> <!-- default -->
<availability distr="gamma" CV="2"/>
<availability distr="lognormal" CV="2"/>
<availability distr="gamma" variance="3"/>
<availability distr="lognormal" CV="3"/>```