Add additional allowed prefixes for booleans

[nusbaume]

Description

This PR updates the set of allowed prefixes for booleans (e.g. variables that are of type logical in Fortran) to include do and is along with flag_for. For example, with these rule changes the following three (made up) standard names would be allowed:

flag_for_deep_convection
do_deep_convective_transport
is_convective_cloud

whereas with the current rule set all three would need to start with flag_for:

flag_for_deep_convection
flag_for_deep_convective_transport
flag_for_convective_cloud

which may not be as clear.

Issues

Addresses the "boolean indicator" discussion found in issue #101

[climbfuji]

Is the idea that we can use either of? Are they / should they be considered equivalent? Otherwise, we need to come up with rules when to use do_, is_, flag_for_ (not a bad thing to have imo).