only allow one version of a module to load at a time

The modulefiles created by deploy-tools allow multiple versions of the same module to be loaded at the same time. This is counter productive as they will most likely be altering the same environment variables and clash with each other.

deploy-tools should instead emulate what the python modules at DLS do. That is, loading a new version of python automatically unloads any currently loaded version.

e.g. two dls-pmac-control show in the following sequence:
```bash
hgv27681@pc0116: ~
$ module load dls-pmac-control/0.1 [8:39:03]

hgv27681@pc0116: ~
$ module load dls-pmac-control/0.2 [8:39:12]

hgv27681@pc0116: ~
$ module list [8:39:15]
Currently Loaded Modulefiles:
1) use.own 3) controls 5) epics/3.14.12.7 7) dls-pmac-control/0.2
2) controls-tools 4) controls_dev 6) dls-pmac-control/0.1(default)
```