add default ctor and reset method to ModelLoader

[missirol]

Small improvements to the ModelLoader class.

• Add a default value for the model-name argument of its ctor.
• Add a reset method to be used to (a) close the open shared library if needed, and (b) specify the name of a new shared library to be loaded.
• Move the addition of ".so" to the library name to the load_model method, so that the new method model_name() returns the model name specified in ctor or reset.
• Minor update to the Makefile so that running make clean multiple times in a row does not lead to an error.

Backward compatible, and necessary for cms-sw/cmssw#47070 (see cms-sw/cmssw#46740).

[aloeliger]

@missirol Could you also create a PR for v1.1.4 to the externals to include this PR with cms-sw/cmssw#47070?

[missirol]

Okay, this is in cms-sw/cmsdist#9606.