Add a "light" installation of msspec

This is similar to a "devel" installation, but only the
virtualenv is created and the msspec package is installed
inside (not in edit mode). Bindings to the Fortran code
are not built.

It is intended to use msspec functions to create clusters but
without having to install wx.

Makefile

@@ -32,6 +32,11 @@ devel: venv pybinding wx
 	@$(INSIDE_VENV) pip install -e src/
 
 
+light: VENV_PATH = ./_venv
+light: venv
+	@$(INSIDE_VENV) pip install src/
+
+
 _build_wx/wxPython.target:
 	@$(INSIDE_VENV) echo "Building wxPython for your `python --version 2>&1` under Linux $(DISTRO_RELEASE)..."
 	# Create a folder to build wx into

src/msspec/utils.py

@@ -38,7 +38,11 @@ import ase.atom
 from ase import Atom
 from ase import Atoms
 
-from msspec.iodata import Data
+try:
+    from msspec.iodata import Data
+except ImportError as err:
+    print(err)
+
 from msspec.misc import LOGGER