Support ModelDB entries that contain multiple models

[olupton]

For example, https://github.com/ModelDBRepository/97917 contains two sets of mechanisms:

• https://github.com/ModelDBRepository/97917/tree/03957add683174b5c2fd73bc2e79c8018e752eef/nrntraub/mod
• https://github.com/ModelDBRepository/97917/tree/03957add683174b5c2fd73bc2e79c8018e752eef/pardentategyrus

which cannot be linked into a single special because the SUFFIXes are not unique:

• https://github.com/ModelDBRepository/97917/blob/03957add683174b5c2fd73bc2e79c8018e752eef/nrntraub/mod/cat.mod#L17
• https://github.com/ModelDBRepository/97917/blob/03957add683174b5c2fd73bc2e79c8018e752eef/pardentategyrus/tca.mod#L24

At present the HOC scripts try to compile mechanisms on the fly, which is doubly problematic because:

• the output from nrnivmodl is not deterministic (it contains make -j 4...)
• it thwarts our attempts to record compilation and execution time separately (Add runtimes report for nrnivmodl & model runtimes #64)

It might be better to support multiple models within a single ModelDB ID at a higher level.

[olupton]

Note that the changes in #89 flagged many models that contain multiple independent sets of .mod files within a single ID.